2016 Rio Olympics - Tennis - Preliminary - Women's Singles Second Round - Olympic Tennis Centre - Rio de Janeiro, Brazil - 08/08/2016. Petra Kvitova (CZE) of Czech Republic in action against Caroline Wozniacki (DEN) of Denmark. REUTERS/Toby Melville

2016 Rio Olympics - Tennis - Preliminary - Women's Singles Second Round - Olympic Tennis Centre - Rio de Janeiro, Brazil - 08/08/2016. Caroline Wozniacki (DEN) of Denmark in action against Petra Kvitova (CZE) of Czech Republic. REUTERS/Toby Melville

Michael Phelps of the US is seen with a red cupping mark on his shoulder as he competes in the Men's 4 x 100m Freestyle Relay Final at the 2016 Rio Olympics in Rio de Janeiro, Brazil August 7, 2016. . REUTERS/Dominic Ebenbichler/File Photo

Migrants play table football at a makeshift camp in Via Cupa (Gloomy Street) in downtown Rome, Italy, August 1, 2016. REUTERS/Max Rossi

Migrants play soccer at makeshift camp in Via Cupa (Gloomy Street) in downtown Rome, Italy, August 2, 2016. REUTERS/Max Rossi

2016 Rio Olympics - Swimming - Victory Ceremony - Men's 4 x 100m Freestyle Relay Victory Ceremony - Olympic Aquatics Stadium - Rio de Janeiro, Brazil - 08/08/2016. Michael Phelps (USA), Ryan Held (USA), Caeleb Dressel (USA) and Nathan Adrian (USA) of USA pose with their medals REUTERS/David Gray

2016 Rio Olympics - Tennis - Preliminary - Men's Singles First Round - Olympic Tennis Centre - Rio de Janeiro, Brazil - 07/08/2016. Novak Djokovic (SRB) of Serbia reacts during his match against Juan Martin Del Potro (ARG) of Argentina. REUTERS/Toby Melville

2016 Rio Olympics - Tennis - Preliminary - Men's Singles First Round - Olympic Tennis Centre - Rio de Janeiro, Brazil - 07/08/2016. Novak Djokovic (SRB) of Serbia reacts after losing his match against Juan Martin Del Potro (ARG) of Argentina. REUTERS/Toby Melville