Naomi Osaka, the champion of U.S. Open women's singles, smiles during a press conference in Yokohama. Osaka defeated Serena Williams of the U.S. on Saturday, Sept. 8, to become th...

Novak Djokovic, of Serbia, holds the trophy after defeating Juan Martin del Potro, of Argentina, during the men's final of the U.S. Open tennis tournament, in New York.

Novak Djokovic, of Serbia, celebrates after defeating Juan Martin del Potro, of Argentina, in the men's final of the U.S. Open tennis tournament, in New York.

Novak Djokovic, of Serbia, celebrates after defeating Juan Martin del Potro, of Argentina, in the men's final of the U.S. Open tennis tournament, in New York.

Rafael Nadal, of Spain, challenges a call during the second set against Juan Martin del Potro, of Argentina, during the semifinals of the U.S. Open tennis tournament

Novak Djokovic, of Serbia, celebrates after defeating Kei Nishikori, of Japan, during the semifinals of the U.S. Open tennis tournament.

Serena Williams returns a shot to Anastasija Sevastova, of Latvia, during the semifinals of the U.S. Open tennis tournament, in New York.

Madison Keys returns a shot to Naomi Osaka, of Japan, during the semifinals of the U.S. Open tennis tournament, in New York.

Naomi Osaka, of Japan, celebrates after defeating Madison Keys in the semifinals of the U.S. Open tennis tournament, in New York.

David Wagner returns a shot during a practice session for the wheelchair competition at the U.S. Open tennis tournament, in New York.

Kgothatso "KG" Montjane returns a shot during a practice session for the wheelchair competition at the U.S. Open tennis tournament, in New York.

Madison Keys celebrates after defeating Carla Suarez Navarro, of Spain, in the quarterfinals of the U.S. Open tennis tournament, in New York.

Carla Suarez Navarro, of Spain, hits a forehand to Madison Keys, of the United States, during the quarterfinals of the U.S. Open tennis tournament in New York.

Marin Cilic, of Croatia, returns a shot to Kei Nishikori, of Japan, during the quarterfinals of the U.S. Open tennis tournament, in New York.

Kei Nishikori, of Japan, reaches to return the ball to Marin Cilic, of Croatia, during the quarterfinals of the U.S. Open tennis tournament in New York.

Naomi Osaka, of Japan, returns a shot to Lesia Tsurenko, of Ukraine, during the quarterfinals of the U.S. Open tennis tournament, in New York.

Rafael Nadal, of Spain, celebrates after defeating Dominic Thiem, of Austria, during the quarterfinals of the U.S. Open tennis tournament, in New York.

Novak Djokovic, of Serbia, reaches for a shot from John Millman, of Australia, during the quarterfinals of the U.S. Open tennis tournament in New York.

John Millman, of Australia, gestures during a match against Novak Djokovic, of Serbia, during the quarterfinals of the U.S. Open tennis tournament in New York.

Dominic Thiem, of Austria, tosses the ball for a serve to Rafael Nadal, of Spain, during the quarterfinals of the U.S. Open tennis tournament in New York.