That is also why Djokovic is not expected to be able to enter the U.S. Open, the year's last Grand Slam tournament, which begins in New York on Aug. 29.

Djokovic is a 35-year-old from Serbia who owns 21 major championships, one behind Rafael Nadal for the men's record. Djokovic has said he won't get vaccinated against the illness caused by the coronavirus, even if that prevents him from playing in certain tournaments. He missed the Australian Open in January after being deported from that country and needed to sit out two events in the United States earlier this year, along with a tournament in Montreal this week.

Unvaccinated foreign citizens can't go to Canada or the U.S. currently; the Cincinnati tournament's news release announcing Djokovic's withdrawal cited "travel restrictions."

Djokovic has said he was still holding out hope of being able to enter the U.S. Open, but that would require a change of government policy, which the U.S. Tennis Association says it will adhere to.

Djokovic is a three-time champion at the U.S. Open and was the runner-up last year to Daniil Medvedev.

Other men who have withdrawn from the field in Cincinnati include defending champion Alexander Zverev – the 2020 U.S. Open runner-up tore ankle ligaments at the French Open in June – and injured players Gael Monfils, Reilly Opelka, Oscar Otte and Dominic Thiem. Australian Open finalist Danielle Collins pulled out of the women's draw because of a neck injury, while three-time major champion Angelique Kerber made a scheduling change and opted not to compete in Cincinnati.