Angelique Kerber, of Germany, returns a shot to Anhelina Kalinina, of Ukraine, during the second round of the US Open tennis championships, Thursday, Sept. 2, 2021, in New York. (AP Photo/Elise Amendola)

Lauren Davis, right, of the United States, returns a shot to Bianca Andreescu, of Canada, during the second round of the US Open tennis championships, Thursday, Sept. 2, 2021, in New York. (AP Photo/Frank Franklin II)

Kei Nishikori, of Japan, returns a shot to Mackenzie McDonald, of the United States, during the second round of the US Open tennis championships, Thursday, Sept. 2, 2021, in New York. (AP Photo/Frank Franklin II)

Alexander Zverev, of Germany, returns a shot to Albert Ramos-Vinolas, of Spain, during the second round of the US Open tennis championships, Thursday, Sept. 2, 2021, in New York. (AP Photo/Elise Amendola)

Ashleigh Barty, of Australia, returns a shot to Clara Tauson, of Denmark, during the second round of the US Open tennis championships, Thursday, Sept. 2, 2021, (AP Photo/Elise Amendola)

Karolina Pliskova, of the Czech Republic, reacts after winning a point against Amanda Anisimova, of the United States, during the second round of the US Open tennis championships, Thursday, Sept. 2, 2021, in New York. (AP Photo/Frank Franklin II)

Amanda Anisimova, of the United States, reacts after scoring a point against Karolina Pliskova, of the Czech Republic, during the second round of the US Open tennis championships, Thursday, Sept. 2, 2021, in New York. (AP Photo/Frank Franklin II)

Denis Shapovalov, of Canada, reacts after winning a point against Roberto Carballes Baena, of Spain, during the second round of the US Open tennis championships, Thursday, Sept. 2, 2021, in New York. (AP Photo/Frank Franklin II)

Novak Djokovic, of Serbia, returns a shot to Tallon Griekspoor, of the Netherlands, during the second round of the US Open tennis championships, Thursday, Sept. 2, 2021, in New York. (AP Photo/Frank Franklin II)

Ashleigh Barty, of Australia, serves to Clara Tauson, of Denmark, during the second round of the US Open tennis championships, Thursday, Sept. 2, 2021, in New York. (AP Photo/Elise Amendola)

Oscar Otte, of Germany, returns a shot to Lorenzo Sonego, of Italy, during the first round of the US Open tennis championships, Tuesday, Aug. 31, 2021, in New York. (AP Photo/Seth Wenig)