Serena Williams of the U.S.A lifts the trophy after winning her Women's Final match against Garbine Muguruza of Spain at the Wimbledon Tennis Championships in London, July 11, 2015. REUTERS/Toby Melville 

Serena Williams of the U.S.A celebrates after winning her Women's Final match against Garbine Muguruza of Spain at the Wimbledon Tennis Championships in London, July 11, 2015. REUTERS/Suzanne Plunkett 

Serena Williams of the U.S. poses with her trophy and her dog named Chip in the dressing room after winning the women's singles final match against Lucie Safarova of the Czech Republic during the French Open tennis tournament at the Roland Garros stadium in Paris, France, June 6, 2015. REUTERS/Corinne Dubreuil/FFT/Pool 

Serena Williams of the U.S. poses with her trophy after defeating Maria Sharapova of Russia in their women's singles final match at the Australian Open 2015 tennis tournament in Melbourne January 31, 2015. REUTERS/Brandon Malone 

Caroline Wozniacki of Denmark holds her runner up trophy and shares a laugh with Serena Williams of the U.S. after Williams won their women's singles finals match at the 2014 U.S. Open tennis tournament in New York, September 7, 2014. REUTERS/Eduardo Munoz 

Serena Williams of the U.S. celebrates after defeating Caroline Wozniacki of Denmark in their women's singles finals match at the 2014 U.S. Open tennis tournament in New York, September 7, 2014. REUTERS/Shannon Stapleton 

Serena Williams of the U.S. raises her trophy after defeating Victoria Azarenka of Belarus (L, holding runner up trophy) in their women's singles final match at the U.S. Open tennis championships in New York September 8, 2013. REUTERS/Mike Segar 

Serena Williams of the U.S. shakes hands with Maria Sharapova (R) of Russia after winning their women's singles final match at the French Open tennis tournament at the Roland Garros stadium in Paris June 8, 2013. REUTERS/Vincent Kessler 

Serena Williams of the U.S. celebrates defeating Maria Sharapova of Russia in their women's singles final match to win the French Open tennis tournament at the Roland Garros stadium in Paris June 8, 2013. REUTERS/Philippe Wojazer 

Serena Williams of the U.S. poses with her trophy after defeating Victoria Azarenka of Belarus in their women's singles finals match at the U.S. Open tennis tournament in New York September 9, 2012. REUTERS/Kevin Lamarque 

Gold medallist Serena Williams of the U.S. poses after winning the women's singles gold medal match against Russia's Maria Sharapova, during the presentation ceremony at the All England Lawn Tennis Club during the London 2012 Olympic Games August 4, 2012. REUTERS/Stefan Wermuth 

Serena Williams of the U.S. prepares to hit a return to Agnieszka Radwanska of Poland during their women's final tennis match at the Wimbledon tennis championships in London July 7, 2012. Williams went on the win the match. REUTERS/Dylan Martinez 

Serena Williams of the U.S. runs through the crowd to see her father and sister after defeating Agnieszka Radwanska of Poland in their women's final tennis match at the Wimbledon tennis championships in London July 7, 2012. REUTERS/Dylan Martinez 

Serena Williams of the U.S. (L) slaps hands with Venus Williams of the U.S. during their women's doubles tennis match against Andrea Hlavackova of the Czech Republic and Lucie Hradecka of the Czech Republic, which they went on to win, at the Wimbledon tennis championships in London July 7, 2012. REUTERS/Stefan Wermuth 

Serena Williams of the U.S. celebrates defeating Russia's Vera Zvonareva in the womens' singles final at the 2010 Wimbledon tennis championships in London, July 3, 2010. REUTERS/Stefan Wermuth 

Serena Williams of the U.S. winks during the awarding ceremony after winning her women's singles final match against Belgium's Justine Henin (L) at the Australian Open tennis tournament in Melbourne January 30, 2010. REUTERS/Vivek Prakash 

Serena Williams of the U.S. holds her trophy after defeating Venus Williams of the U.S. in their Ladies' Singles finals match at the Wimbledon tennis championships in London, July 4, 2009. REUTERS/Toby Melville 

Serena Williams (L) of the U.S. and Russia's Dinara Safina pose with their trophies after Williams won their women's singles final match at the Australian Open tennis tournament in Melbourne January 31, 2009. REUTERS/Darren Whiteside 

Serena Williams of the U.S. returns a shot to Jelena Jankovic of Serbia during the women's final match, which she went on to win, at the U.S. Open tennis tournament at Flushing Meadows in New York September 7, 2008. REUTERS/Jeff Haynes 

Serena Williams of the US holds 2005 Australian Open tennis trophy after her win over compatriot Lindsay Davenport in Melbourne. REUTERS/Adrees Latif 

Serena Williams (L) of the United States blows kisses to the crowdafter defeating her sister Venus (R) in the women's final at theAustralian Open in Melbourne January 25, 2003. Serena defeated Venus7-6 3-6 6-4. REUTERS/Mark BakerPJ/CP 

Serena Williams of the U.S. returns to Belgium's Els Callens at theWimbledon tennis championships, June 28, 2002. Williams went on to win thetournament, beating her sister Venus in the final. REUTERS/Jeff J Mitchell 

Venus Williams (L) of the United States and sister Serena Williams laugh as they celebrate their doubles win at the U.S. Open in Flushing Meadows September 12, 1999. The Williams sisters defeated Sandrine Testud of France and Chandra Rubin of the United States 4-6 6-1 6-4 to win the title. 

The United States Serena Williams returns a shot on her way to a 6-3 3-6 7-5 victory of Germany's Steffi Graf in the finals of the Evert Cup in Indian Wells, California, March 13. Williams wins $200,000 for the event. 

The United States Serena Williams (L) and opponent, Germany's Steffi Graf pose for photographers after Wiliams 6-3 3-6 7-5 victory in the finals of the Evert Cup in Indian Wells, California March 13.