Caroline Wozniacki of Denmark holds a towel over her face during her match with Petra Cetkovska of Czech Republic at the U.S. Open Championships tennis tournament in New York, September 3, 2015. REUTERS/Shannon Stapleton 

Petra Cetkovska of Czech Republic celebrates her win over Caroline Wozniacki of Denmark during their second round match at the U.S. Open Championships tennis tournament in New York, September 4, 2015. REUTERS/Shannon Stapleton 

Petra Cetkovska of Czech Republic celebrates her win over Caroline Wozniacki of Denmark during their second round match at the U.S. Open Championships tennis tournament in New York, September 4, 2015. REUTERS/Shannon Stapleton 

Petra Cetkovska of Czech Republic celebrates her win over Caroline Wozniacki of Denmark during their second round match at the U.S. Open Championships tennis tournament in New York, September 4, 2015. REUTERS/Shannon Stapleton 

Petra Cetkovska of Czech Republic celebrates her win over Caroline Wozniacki of Denmark during their second round match at the U.S. Open Championships tennis tournament in New York, September 4, 2015. REUTERS/Shannon Stapleton 

Petra Cetkovska of Czech Republic celebrates her win over Caroline Wozniacki of Denmark during their second round match at the U.S. Open Championships tennis tournament in New York, September 4, 2015. REUTERS/Shannon Stapleton 

Petra Cetkovska of Czech Republic celebrates her win over Caroline Wozniacki of Denmark during their second round match at the U.S. Open Championships tennis tournament in New York, September 4, 2015.REUTERS/Shannon Stapleton 

Petra Cetkovska of the Czech Republic returns a serve from Denmark's Caroline Wozniacki during their second round match at the U.S. Open Championships tennis tournament in New York, September 3, 2015. REUTERS/Shannon Stapleton 

Caroline Wozniacki of Denmark serves to Petra Cetkovska of the Czech Republic during their second round match at the U.S. Open Championships tennis tournament in New York, September 3, 2015. REUTERS/Carlo Allegri

Petra Cetkovska of the Czech Republic serves to Caroline Wozniacki of Denmark during their second round match at the U.S. Open Championships tennis tournament in New York, September 3, 2015. REUTERS/Carlo Allegri

Caroline Wozniacki of Denmark runs to retun a shot to Petra Cetkovska of the Czech Republic during their second round match at the U.S. Open Championships tennis tournament in New York, September 3, 2015. REUTERS/Carlo Allegri

Caroline Wozniacki of Denmark serves to Petra Cetkovska of the Czech Republic during their second round match at the U.S. Open Championships tennis tournament in New York, September 3, 2015. REUTERS/Carlo Allegri

Petra Cetkovska of the Czech Republic reacts during the women's singles match against Misaki Doi of Japan at the French Open tennis tournament at the Roland Garros stadium in Paris, France, May 24, 2015. REUTERS/Pascal Rossignol

Misaki Doi of Japan reacts during the women's singles match against Petra Cetkovska of the Czech Republic at the French Open tennis tournament at the Roland Garros stadium in Paris, France, May 24, 2015. REUTERS/Pascal Rossignol

Misaki Doi of Japan plays a shot to Petra Cetkovska of the Czech Republic during their women's singles match at the French Open tennis tournament at the Roland Garros stadium in Paris, France, May 24, 2015. REUTERS/Pascal Rossignol

Misaki Doi of Japan plays a shot to Petra Cetkovska of the Czech Republic during their women's singles match at the French Open tennis tournament at the Roland Garros stadium in Paris, France, May 24, 2015. REUTERS/Pascal Rossignol

Misaki Doi of Japan plays a shot to Petra Cetkovska of the Czech Republic during their women's singles match at the French Open tennis tournament at the Roland Garros stadium in Paris, France, May 24, 2015. REUTERS/Pascal Rossignol

Misaki Doi of Japan plays a shot to Petra Cetkovska of the Czech Republic during their women's singles match at the French Open tennis tournament at the Roland Garros stadium in Paris, France, May 24, 2015. REUTERS/Pascal Rossignol

Petra Cetkovska of the Czech Republic plays a shot to Misaki Doi of Japan during their women's singles match at the French Open tennis tournament at the Roland Garros stadium in Paris, France, May 24, 2015. REUTERS/Pascal Rossignol

The official Roland Garros 2015 match ball is seen on the court during the women's singles match between Petra Cetkovska of the Czech Republicand Misaki Doi of Japan at the French Open tennis tournament at the Roland Garros stadium in Paris, France, May 24, 2015. REUTERS/Pascal Rossignol

Petra Cetkovska of the Czech Republic plays a shot to Misaki Doi of Japan during their women's singles match at the French Open tennis tournament at the Roland Garros stadium in Paris, France, May 24, 2015. REUTERS/Pascal Rossignol

Petra Kvitova of the Czech Republic signs autographs after defeating compatriot Petra Cetkovska during their match at the 2014 U.S. Open tennis tournament in New York, August 28, 2014. REUTERS/Adam Hunger (UNITED STATES - Tags: SPORT TENNIS)

Petra Kvitova of the Czech Republic hits a return to compatriot Petra Cetkovska during their match at the 2014 U.S. Open tennis tournament in New York, August 28, 2014. REUTERS/Adam Hunger (UNITED STATES - Tags: SPORT TENNIS)

Petra Kvitova of the Czech Republic serves to compatriot Petra Cetkovska during their match at the 2014 U.S. Open tennis tournament in New York, August 28, 2014. REUTERS/Adam Hunger (UNITED STATES - Tags: SPORT TENNIS)

Petra Kvitova of the Czech Republic sets up to return to compatriot Petra Cetkovska during their match at the 2014 U.S. Open tennis tournament in New York, August 28, 2014. REUTERS/Adam Hunger (UNITED STATES - Tags: SPORT TENNIS)

Petra Kvitova of the Czech Republic serves to compatriot Petra Cetkovska during their match at the 2014 U.S. Open tennis tournament in New York, August 28, 2014. REUTERS/Adam Hunger (UNITED STATES - Tags: SPORT TENNIS)

Petra Kvitova of the Czech Republic serves to compatriot Petra Cetkovska during their match at the 2014 U.S. Open tennis tournament in New York, August 28, 2014. REUTERS/Adam Hunger (UNITED STATES - Tags: SPORT TENNIS)

Petra Kvitova of the Czech Republic serves to compatriot Petra Cetkovska during their match at the 2014 U.S. Open tennis tournament in New York, August 28, 2014. REUTERS/Adam Hunger (UNITED STATES - Tags: SPORT TENNIS)

Alize Cornet of France hits a return to Petra Cetkovska of the Czech Republic during their women's singles tennis match at the Wimbledon Tennis Championships, in London June 26, 2014. REUTERS/Max Rossi (BRITAIN - Tags: SPORT TENNIS)

Alize Cornet of France hits a return to Petra Cetkovska of the Czech Republic during their women's singles tennis match at the Wimbledon Tennis Championships, in London June 26, 2014. REUTERS/Max Rossi (BRITAIN - Tags: SPORT TENNIS)

Angelique Kerber of Germany returns the ball to Petra Cetkovska of Czech Republic during their women's singles quarter-final match at the Qatar Open tennis tournament in Doha February 14, 2014. Kerber beat Cetkovska 6-1, 6-0. REUTERS/Fadi Al-Assaad (QATAR - Tags: SPORT TENNIS) 

Angelique Kerber of Germany returns the ball to Petra Cetkovska of Czech Republic during their women's singles quarter-final match at the Qatar Open tennis tournament in Doha February 14, 2014. Kerber beat Cetkovska 6-1, 6-0. REUTERS/Fadi Al-Assaad (QATAR - Tags: SPORT TENNIS) 

Petra Cetkovska of Czech Republic returns the ball to Angelique Kerber of Germany during their women's singles quarter-finals match at the Qatar Open tennis tournament in Doha February 14, 2014. Cetkovska lost the match to Kerber. REUTERS/Fadi Al-Assaad (QATAR - Tags: SPORT TENNIS) 

Petra Cetkovska of Czech Republic returns the ball to Angelique Kerber of Germany during their women's singles quarter-finals match at the Qatar Open tennis tournament in Doha February 14, 2014. Cetkovska lost the match to Kerber. REUTERS/Fadi Al-Assaad (QATAR - Tags: SPORT TENNIS) 

Petra Cetkovska of Czech Republic returns the ball to Angelique Kerber of Germany during their women's singles quarter-finals match at the Qatar Open tennis tournament in Doha February 14, 2014. Cetkovska lost the match to Kerber. REUTERS/Fadi Al-Assaad (QATAR - Tags: SPORT TENNIS) 

Angelique Kerber of Germany celebrates defeating Petra Cetkovska of Czech Republic during their women's singles quarter-final match at the Qatar Open tennis tournament in Doha February 14, 2014. REUTERS/Fadi Al-Assaad (QATAR - Tags: SPORT TENNIS) 

Angelique Kerber of Germany celebrates defeating Petra Cetkovska of Czech Republic during their women's singles quarter-final match at the Qatar Open tennis tournament in Doha February 14, 2014. REUTERS/Fadi Al-Assaad (QATAR - Tags: SPORT TENNIS) 

Angelique Kerber of Germany celebrates defeating Petra Cetkovska of Czech Republic during their women's singles quarter-final match at the Qatar Open tennis tournament in Doha February 14, 2014. REUTERS/Fadi Al-Assaad (QATAR - Tags: SPORT TENNIS) 

Li Na of China serves the ball to Petra Cetkovska of Czech Republic during their women's singles match at the Qatar Open tennis tournament in Doha February 13, 2014. REUTERS/Fadi Al-Assaad (QATAR - Tags: SPORT TENNIS) 

Li Na of China serves the ball to Petra Cetkovska of Czech Republic during their women's singles match at the Qatar Open tennis tournament in Doha February 13, 2014. REUTERS/Fadi Al-Assaad (QATAR - Tags: SPORT TENNIS) 

Li Na of China returns the ball to Petra Cetkovska of Czech Republic during their women's singles match at the Qatar Open tennis tournament in Doha February 13, 2014. REUTERS/Fadi Al-Assaad (QATAR - Tags: SPORT TENNIS) 

Li Na of China returns the ball to Petra Cetkovska of Czech Republic during their women's singles match at the Qatar Open tennis tournament in Doha February 13, 2014. REUTERS/Fadi Al-Assaad (QATAR - Tags: SPORT TENNIS) 

Petra Cetkovska of Czech Republic returns the ball to Li Na of China during their women's singles match at the Qatar Open tennis tournament in Doha February 13, 2014. REUTERS/Fadi Al-Assaad (QATAR - Tags: SPORT TENNIS) 

Petra Cetkovska of Czech Republic returns the ball to Li Na of China during their women's singles match at the Qatar Open tennis tournament in Doha February 13, 2014. REUTERS/Fadi Al-Assaad (QATAR - Tags: SPORT TENNIS) 

Petra Cetkovska of Czech Republic returns the ball to Li Na of China during their women's singles match at the Qatar Open tennis tournament in Doha February 13, 2014. REUTERS/Fadi Al-Assaad (QATAR - Tags: SPORT TENNIS) 

Petra Cetkovska of Czech Republic returns the ball to Li Na of China during their women's singles match at the Qatar Open tennis tournament in Doha February 13, 2014. REUTERS/Fadi Al-Assaad (QATAR - Tags: SPORT TENNIS) 

Petra Cetkovska of Czech Republic returns the ball to Li Na of China during their women's singles match at the Qatar Open tennis tournament in Doha February 13, 2014. REUTERS/Fadi Al-Assaad (QATAR - Tags: SPORT TENNIS) 

Li Na of China returns the ball to Petra Cetkovska of Czech Republic during their women's singles match at the Qatar Open tennis tournament in Doha February 13, 2014. REUTERS/Fadi Al-Assaad (QATAR - Tags: SPORT TENNIS) 

Li Na of China returns the ball to Petra Cetkovska of Czech Republic during their women's singles match at the Qatar Open tennis tournament in Doha February 13, 2014. REUTERS/Fadi Al-Assaad (QATAR - Tags: SPORT TENNIS) 

Li Na of China returns the ball to Petra Cetkovska of Czech Republic during their women's match at the Qatar Open tennis tournament in Doha February 13, 2014. REUTERS/Fadi Al-Assaad (QATAR - Tags: SPORT TENNIS) 

Sloane Stephens of the U.S. hits a return to Petra Cetkovska of the Czech Republic in their women's singles tennis match at the Wimbledon Tennis Championships, in London June 29, 2013. REUTERS/Stefan Wermuth (BRITAIN - Tags: SPORT TENNIS)

Sloane Stephens of the U.S. hits a return to Petra Cetkovska of the Czech Republic in their women's singles tennis match at the Wimbledon Tennis Championships, in London June 29, 2013. REUTERS/Stefan Wermuth (BRITAIN - Tags: SPORT TENNIS)

Sloane Stephens of the U.S. hits a return to Petra Cetkovska of the Czech Republic in their women's singles tennis match at the Wimbledon Tennis Championships, in London June 29, 2013. REUTERS/Stefan Wermuth (BRITAIN - Tags: SPORT TENNIS)

Sloane Stephens of the U.S. celebrates after defeating Petra Cetkovska of the Czech Republic in their women's singles tennis match at the Wimbledon Tennis Championships, in London June 29, 2013. REUTERS/Stefan Wermuth (BRITAIN - Tags: SPORT TENNIS)

Sloane Stephens of the U.S. celebrates after defeating Petra Cetkovska of the Czech Republic in their women's singles tennis match at the Wimbledon Tennis Championships, in London June 29, 2013. REUTERS/Stefan Wermuth (BRITAIN - Tags: SPORT TENNIS)

Sloane Stephens of the U.S. celebrates after defeating Petra Cetkovska of the Czech Republic in their women's singles tennis match at the Wimbledon Tennis Championships, in London June 29, 2013. REUTERS/Stefan Wermuth (BRITAIN - Tags: SPORT TENNIS)

Sloane Stephens of the U.S. hits a return to Petra Cetkovska of the Czech Republic in their women's singles tennis match at the Wimbledon Tennis Championships, in London June 28, 2013. REUTERS/Suzanne Plunkett (BRITAIN - Tags: SPORT TENNIS)

Sloane Stephens of the U.S. listens through her earphones before her women's singles tennis match against Petra Cetkovska of the Czech Republic at the Wimbledon Tennis Championships, in London June 28, 2013. REUTERS/Suzanne Plunkett (BRITAIN - Tags: SPORT TENNIS)

Sloane Stephens of the U.S. reacts after missing a shot during her women's singles tennis match against Petra Cetkovska of the Czech Republic at the Wimbledon Tennis Championships, in London June 28, 2013. REUTERS/Suzanne Plunkett (BRITAIN - Tags: SPORT TENNIS)

Sloane Stephens of the U.S. hits a return to Petra Cetkovska of the Czech Republic in their women's singles tennis match at the Wimbledon Tennis Championships, in London June 28, 2013. REUTERS/Suzanne Plunkett (BRITAIN - Tags: SPORT TENNIS)