Serbia's Novak Djokovic, left, speaks with his wife Jelena during a final match of the Adria Tour charity tournament, between Austria's Dominic Thiem and Serbia's Filip Krajinovic, in Belgrade, Serbia, Sunday, June 14, 2020. (AP Photo/Darko Vojinovic)

Fans watch a tennis match between Serbia's Novak Djokovic and Germany's Alexander Zverev, of the Adria Tour charity tournament in Belgrade, Serbia, Sunday, June 14, 2020. (AP Photo/Darko Vojinovic)

In this photo taken on Sunday, June 14, 2020, Serbia's Novak Djokovic, centre, poses with volunteers and players after the Adria Tour charity tournament in Belgrade, Serbia. Djokovic’s charity tennis exhibition series combined with an overall softening of coronavirus restrictions in Serbia has led to a spike in the number the positive cases among professional athletes. Two tennis players ranked among the top 40 in the world and five players at Serbia’s biggest soccer club have tested positive for the virus after being involved in sporting events where fans packed into the stands and social distancing was not enforced. (AP Photo/Darko Vojinovic)

In this photo taken on Friday, June 12, 2020, Serbia's Novak Djokovic, left, and Bulgaria's Grigor Dimitrov attend a press conference prior a a charity tournament Adria Tour, in Belgrade, Serbia. Dimitrov has tested positive for COVID-19, it is announced Monday June 22, 2020, leading to the cancellation of an exhibition event in Croatia where top-ranked Novak Djokovic was due to play in the final. Organizers said Sunday the exhibition has now been canceled. (AP Photo/Darko Vojinovic)

In this photo taken on Thursday June 18, 2020, Bulgarian tennis player Grigor Dimitrov, rear, plays basketball with Serbia's Novak Djokovic in Zadar, Croatia. Dimitrovsays he has tested positive for COVID-19 and his announcement led to the cancellation of an exhibition event in Croatia where Novak Djokovic was scheduled to play on Sunday, June 21. (AP Photo/Zvonko Kucelin)

In this photo taken on Sunday, June 14, 2020, Serbia's Novak Djokovic, centre, poses with volunteers and players after the Adria Tour charity tournament in Belgrade, Serbia. Djokovic’s charity tennis exhibition series combined with an overall softening of coronavirus restrictions in Serbia has led to a spike in the number the positive cases among professional athletes. Two tennis players ranked among the top 40 in the world and five players at Serbia’s biggest soccer club have tested positive for the virus after being involved in sporting events where fans packed into the stands and social distancing was not enforced. (AP Photo/Darko Vojinovic)

Spectators watch a tennis match during an exhibition tournament in Zadar, Croatia, Sunday, June 21, 2020. Tennis player Grigor Dimitrov says he has tested positive for COVID-19 and his announcement led to the cancellation of an exhibition event in Croatia where Novak Djokovic was scheduled to play on Sunday. (AP Photo/Zvonko Kucelin)

Serbia's Novak Djokovic returns the ball during an exhibition tournament in Zadar, Croatia, Sunday, June 21, 2020. Tennis player Grigor Dimitrov says he has tested positive for COVID-19 and his announcement led to the cancellation of an exhibition event in Croatia where Novak Djokovic was scheduled to play on Sunday. (AP Photo/Zvonko Kucelin)

Germany's Alexander Zverev reacts during the match against Serbia's Novak Djokovic, of the Adria Tour charity tournament in Belgrade, Serbia, Sunday, June 14, 2020. (AP Photo/Darko Vojinovic)