Alexander Zverev, of Germany, returns a shot to Sam Querrey, of the United States, during the first round of the US Open tennis championships, Tuesday, Aug. 31, 2021, in New York. (AP Photo/John Minchillo)

Holger Vitus Nodskov Rune of Denmark, reaches for a ball from Novak Djokovic, of Serbia, during the first round of the US Open tennis championships, Tuesday, Aug. 31, 2021, in New York. (AP Photo/Frank Franklin II)

Ashleigh Barty, of Australia, returns a shot to Vera Zvonareva, of Russia, during the first round of the US Open tennis championships, Tuesday, Aug. 31, 2021, in New York. (AP Photo/John Minchillo)

Alexander Zverev, of Germany, returns a shot to Sam Querrey, of the United States, during the first round of the US Open tennis championships, Tuesday, Aug. 31, 2021, in New York. (AP Photo/John Minchillo)

Maxime Cressy, of the United States, returns a shot to Pablo Carreno Busta, of Spain, during the first round of the US Open tennis championships, Tuesday, Aug. 31, 2021, in New York. (AP Photo/Seth Wenig)

Bianca Andreescu, of Canada, reacts between points during the first round of the US Open tennis championships, Tuesday, Aug. 31, 2021, in New York. (AP Photo/Frank Franklin II)

Novak Djokovic, of Serbia, returns a shot during the first round of the US Open tennis championships, Tuesday, Aug. 31, 2021, in New York. (AP Photo/Frank Franklin II)

Ashleigh Barty, of Australia, waits during a break in play against Vera Zvonareva, of Russia, during the first round of the US Open tennis championships, Tuesday, Aug. 31, 2021, in New York. (AP Photo/John Minchillo)

Novak Djokovic, of Serbia, returns a shot during the first round of the US Open tennis championships, Tuesday, Aug. 31, 2021, in New York. (AP Photo/Frank Franklin II)

Novak Djokovic, of Serbia, returns a shot to during the first round of the US Open tennis championships, Tuesday, Aug. 31, 2021, in New York. (AP Photo/Frank Franklin II)

Holger Vitus Nodskov Rune of Denmark, bottom, serves to Novak Djokovic, of Serbia, during the first round of the US Open tennis championships, Tuesday, Aug. 31, 2021, in New York. (AP Photo/Frank Franklin II)

Holger Vitus Nodskov Rune of Denmark, left, and Novak Djokovic, of Serbia, meet at the net after Djokovic won their first-round match at the US Open tennis championships, Tuesday, Aug. 31, 2021, in New York. (AP Photo/Frank Franklin II)