World number one tennis player Novak Djokovic poses after a news conference on the upcoming Adria Tour tennis tournament, in Belgrade, Serbia, May 25, 2020. REUTERS/Marko Djurica

World number one tennis player Novak Djokovic leaves after a news conference on the upcoming Adria Tour tennis tournament, in Belgrade, Serbia, May 25, 2020. REUTERS/Marko Djurica

World number one tennis player Novak Djokovic speaks at a news conference on the upcoming Adria Tour tennis tournament, in Belgrade, Serbia, May 25, 2020. REUTERS/Marko Djurica

World number one tennis player Novak Djokovic reacts at a news conference on the upcoming Adria Tour tennis tournament, in Belgrade, Serbia, May 25, 2020. REUTERS/Marko Djurica

World number one tennis player Novak Djokovic holds a news conference on the upcoming Adria Tour tennis tournament, in Belgrade, Serbia, May 25, 2020. REUTERS/Marko Djurica

World number one tennis player Novak Djokovic holds a news conference on the upcoming Adria Tour tennis tournament, in Belgrade, Serbia, May 25, 2020. REUTERS/Marko Djurica

World number one tennis player Novak Djokovic holds a news conference on the upcoming Adria Tour tennis tournament, in Belgrade, Serbia, May 25, 2020. REUTERS/Marko Djurica

World number one tennis player Novak Djokovic holds a news conference on the upcoming Adria Tour tennis tournament, in Belgrade, Serbia, May 25, 2020. REUTERS/Marko Djurica

World number one tennis player Novak Djokovic holds a news conference on the upcoming Adria Tour tennis tournament, in Belgrade, Serbia, May 25, 2020. REUTERS/Marko Djurica

World number one tennis player Novak Djokovic holds a news conference on the upcoming Adria Tour tennis tournament, in Belgrade, Serbia, May 25, 2020. REUTERS/Marko Djurica

World number one tennis player Novak Djokovic holds a news conference on the upcoming Adria Tour tennis tournament, in Belgrade, Serbia, May 25, 2020. REUTERS/Marko Djurica

World number one tennis player Novak Djokovic arrives to hold a news conference on the upcoming Adria Tour tennis tournament, in Belgrade, Serbia, May 25, 2020. REUTERS/Marko Djurica

Tennis - All-Czech Tennis Tournament Press Conference - TK Sparta Prague, Prague, Czech Republic, May 25, 2020 Czech Republic's Petra Kvitova after the press conference following the outbreak of the coronavirus disease (COVID-19) REUTERS/David W Cerny

Tennis - All-Czech Tennis Tournament Press Conference - TK Sparta Prague, Prague, Czech Republic, May 25, 2020 Czech Republic's Barbora Strycova during the press conference following the outbreak of the coronavirus disease (COVID-19) REUTERS/David W Cerny

Tennis - All-Czech Tennis Tournament Press Conference - TK Sparta Prague, Prague, Czech Republic, May 25, 2020 Czech Republic's Petra Kvitova during the press conference following the outbreak of the coronavirus disease (COVID-19) REUTERS/David W Cerny

Tennis - All-Czech Tennis Tournament Press Conference - TK Sparta Prague, Prague, Czech Republic, May 25, 2020 Czech Republic's Petra Kvitova and Barbora Strycova during the press conference following the outbreak of the coronavirus disease (COVID-19) REUTERS/David W Cerny

Tennis - All-Czech Tennis Tournament Press Conference - TK Sparta Prague, Prague, Czech Republic, May 25, 2020 Czech Republic's Petra Kvitova and Barbora Strycova during the press conference following the outbreak of the coronavirus disease (COVID-19) REUTERS/David W Cerny

Tennis - All-Czech Tennis Tournament Press Conference - TK Sparta Prague, Prague, Czech Republic, May 25, 2020 Czech Republic's Petra Kvitova and Barbora Strycova during the press conference following the outbreak of the coronavirus disease (COVID-19) REUTERS/David W Cerny

Tennis - All-Czech Tennis Tournament Press Conference - TK Sparta Prague, Prague, Czech Republic, May 25, 2020 Czech Republic's Petra Kvitova during the press conference following the outbreak of the coronavirus disease (COVID-19) REUTERS/David W Cerny

Tennis - All-Czech Tennis Tournament Press Conference - TK Sparta Prague, Prague, Czech Republic, May 25, 2020 Czech Republic's Petra Kvitova during the press conference following the outbreak of the coronavirus disease (COVID-19) REUTERS/David W Cerny

Tennis - All-Czech Tennis Tournament Press Conference - TK Sparta Prague, Prague, Czech Republic, May 25, 2020 Czech Republic's Petra Kvitova and Barbora Strycova during the press conference following the outbreak of the coronavirus disease (COVID-19) REUTERS/David W Cerny

People play tennis at Trinity Bellwoods park as the province prepares for more phased re-openings from the coronavirus disease (COVID-19) restrictions in Toronto, Ontario, Canada May 24, 2020. REUTERS/Carlos Osorio

Local residents play a modified tennis match using lawn chairs at a residential tennis court, amid the coronavirus disease (COVID-19) outbreak in Washington, U.S., May 23, 2020. REUTERS/Tom Brenner

Local residents play a modified tennis match using lawn chairs at a residential tennis court, amid the coronavirus disease (COVID-19) outbreak in Washington, U.S., May 23, 2020. REUTERS/Tom Brenner

People play table tennis on the seafront in Brighton, following the outbreak of the coronavirus disease (COVID-19), Brighton, Britain, May 21, 2020. REUTERS/Matthew Childs

A tennis table with barrier tape is pictured in the courtyard at the College Rosa Parks school during its reopening in Nantes as a small part of French shoolchildren head back to their schools with new rules and social distancing during the outbreak of the coronavirus disease (COVID-19) in France, May 20, 2020. REUTERS/Stephane Mahe

Employees play table tennis at Baidu's headquarters in Beijing, following the novel coronavirus disease (COVID-19) outbreak, China May 18, 2020. REUTERS/Tingshu Wang

A boy plays tennis at Enfield Lawn Tennis Club, following the outbreak of the coronavirus disease (COVID-19), London, Britain, May 13, 2020. REUTERS/John Sibley

People play tennis at Enfield Lawn Tennis Club, following the outbreak of the coronavirus disease (COVID-19), London, Britain, May 13, 2020. REUTERS/John Sibley

A boy plays tennis at Enfield Lawn Tennis Club, following the outbreak of the coronavirus disease (COVID-19), London, Britain, May 13, 2020. REUTERS/John Sibley

A boy plays tennis at Enfield Lawn Tennis Club, following the outbreak of the coronavirus disease (COVID-19), London, Britain, May 13, 2020. REUTERS/John Sibley

A sign is seen at Enfield Lawn Tennis Club informing people they can now play tennis against one other person, following the outbreak of the coronavirus disease (COVID-19), London, Britain, May 13, 2020. REUTERS/John Sibley

A man plays tennis in a park in Newcastle-under-Lyme, following the outbreak of the coronavirus disease (COVID-19), Newcastle-under-Lyme, Britain, May 13, 2020. REUTERS/Carl Recine

A man plays tennis in a park in Newcastle-under-Lyme, following the outbreak of the coronavirus disease (COVID-19), Newcastle-under-Lyme, Britain, May 13, 2020. REUTERS/Carl Recine

A man plays tennis in a park in Newcastle-under-Lyme, following the outbreak of the coronavirus disease (COVID-19), Newcastle-under-Lyme, Britain, May 13, 2020. REUTERS/Carl Recine

Tennis courts are seen in Archbishop's Park, following the outbreak of the coronavirus disease (COVID-19), London, Britain, May 13, 2020. REUTERS/Henry Nicholls

Groundsman Kristian Gregory cleans the courts at St George’s Hill Lawn Tennis Club in Weybridge as they prepare to reopen, following the outbreak of the coronavirus disease (COVID-19), Weybridge, Britain, May 12, 2020. REUTERS/Peter Nicholls

Groundsman Kristian Gregory cleans the courts at St George’s Hill Lawn Tennis Club in Weybridge as they prepare to reopen, following the outbreak of the coronavirus disease (COVID-19), Weybridge, Britain, May 12, 2020. REUTERS/Peter Nicholls

Head groundsman Paul Bishop cuts the grass on the courts at St George’s Hill Lawn Tennis Club in Weybridge as they prepare to reopen, following the outbreak of the coronavirus disease (COVID-19), Weybridge, Britain, May 12, 2020. REUTERS/Peter Nicholls

Head groundsman Paul Bishop cuts the grass on the courts at St George’s Hill Lawn Tennis Club in Weybridge as they prepare to reopen, following the outbreak of the coronavirus disease (COVID-19), Weybridge, Britain, May 12, 2020. REUTERS/Peter Nicholls

Head groundsman Paul Bishop paints lines on the courts at St George’s Hill Lawn Tennis Club in Weybridge as they prepare to reopen, following the outbreak of the coronavirus disease (COVID-19), Weybridge, Britain, May 12, 2020. REUTERS/Peter Nicholls REFILE - CORRECTING CAPTION

Head groundsman Paul Bishop paints lines on the courts at St George’s Hill Lawn Tennis Club in Weybridge as they prepare to reopen, following the outbreak of the coronavirus disease (COVID-19), Weybridge, Britain, May 12, 2020. REUTERS/Peter Nicholls REFILE - CORRECTING CAPTION

Empty tennis courts are seen in Battersea Park, following the outbreak of the coronavirus disease (COVID-19), London, Britain, May 12, 2020. REUTERS/John Sibley

Empty tennis courts are seen in Battersea Park, following the outbreak of the coronavirus disease (COVID-19), London, Britain, May 12, 2020. REUTERS/John Sibley

Empty tennis courts are seen in Battersea Park, following the outbreak of the coronavirus disease (COVID-19), London, Britain, May 12, 2020. REUTERS/John Sibley

Empty tennis courts are seen in Battersea Park, following the outbreak of the coronavirus disease (COVID-19), London, Britain, May 12, 2020. REUTERS/John Sibley

Empty tennis courts are seen in Battersea Park, following the outbreak of the coronavirus disease (COVID-19), London, Britain, May 12, 2020. REUTERS/John Sibley

British tennis player Dominic Inglot during a training session in his garden in Chiswick, following the outbreak of the coronavirus disease (COVID-19), London, Britain, May 12, 2020. REUTERS/Andrew Boyers

British tennis player Dominic Inglot during a training session in his garden in Chiswick, following the outbreak of the coronavirus disease (COVID-19), London, Britain, May 12, 2020. REUTERS/Andrew Boyers

British tennis player Dominic Inglot during a training session in his garden in Chiswick, following the outbreak of the coronavirus disease (COVID-19), London, Britain, May 12, 2020. REUTERS/Andrew Boyers

British tennis player Dominic Inglot during a training session in his garden in Chiswick, following the outbreak of the coronavirus disease (COVID-19), London, Britain, May 12, 2020. REUTERS/Andrew Boyers

British tennis player Dominic Inglot during a training session in his garden in Chiswick, following the outbreak of the coronavirus disease (COVID-19), London, Britain, May 12, 2020. REUTERS/Andrew Boyers

British tennis player Dominic Inglot during a training session in his garden in Chiswick, following the outbreak of the coronavirus disease (COVID-19), London, Britain, May 12, 2020. REUTERS/Andrew Boyers

British tennis player Dominic Inglot during a training session in his garden in Chiswick, following the outbreak of the coronavirus disease (COVID-19), London, Britain, May 12, 2020. REUTERS/Andrew Boyers

British tennis player Dominic Inglot during a training session in his garden in Chiswick, following the outbreak of the coronavirus disease (COVID-19), London, Britain, May 12, 2020. REUTERS/Andrew Boyers

British tennis player Dominic Inglot during a training session in his garden in Chiswick, following the outbreak of the coronavirus disease (COVID-19), London, Britain, May 12, 2020. REUTERS/Andrew Boyers

British tennis player Dominic Inglot during a training session in his garden in Chiswick, following the outbreak of the coronavirus disease (COVID-19), London, Britain, May 12, 2020. REUTERS/Andrew Boyers TPX IMAGES OF THE DAY

A closed notice is seen on the gates to tennis courts in Brockwell Park, following the outbreak of the coronavirus disease (COVID-19), London, Britain, May 12, 2020. REUTERS/John Sibley

Empty tennis courts are seen in Brockwell Park, following the outbreak of the coronavirus disease (COVID-19), London, Britain, May 12, 2020. REUTERS/John Sibley

People play tennis against a wall of the Tate Modern building following the outbreak of the coronavirus disease (COVID-19), London, Britain, May 10, 2020. REUTERS/Henry Nicholls