Sloane Stephens, of the United States, serves to Madison Keys, of the United States, during the first round of the US Open tennis championships, Monday, Aug. 30, 2021, in New York. (AP Photo/Seth Wenig)

Tennis fans walk the grounds of the Billie Jean King National Tennis Center during the first round of the US Open tennis championships, Monday, Aug. 30, 2021, in New York. (AP Photo/Seth Wenig)

Camila Giorgi, of Italy, serves to Simona Halep, of Romania, during the first round of the US Open tennis championships, Monday, Aug. 30, 2021, in New York. (AP Photo/John Minchillo)

Tennis fans line up outside the Billie Jean King National Tennis Center to attend the first round of the US Open tennis championships, Monday, Aug. 30, 2021, in New York. (AP Photo/John Minchillo)

Tennis fans watch as Ricardas Berankis, of Lithuania, serves to Diego Schwartzman, of Argentina, during the first round of the US Open tennis championships, Monday, Aug. 30, 2021, in New York. (AP Photo/John Minchillo)

Tennis fans show their proof of vaccination cards for entry to attend the first round of the US Open tennis championships, Monday, Aug. 30, 2021, in New York. (AP Photo/Seth Wenig)