Change of policy
Djokovic has said he was still holding out hope of being able to enter the US Open, but that would require a change of government policy, which the US Tennis Association says it will adhere to.
Djokovic is a three-time champion at New York and was the runner-up last year to Daniil Medvedev.
Major pull outs
Other men who have withdrawn from the field in Cincinnati include defending champion Alexander Zverev -- the 2020 US Open runner-up tore ankle ligaments at the French Open in June - and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.
Meanwhile, Serena Williams is entered in the field for what is expected to be one of her last appearances on the WTA tour. The American great had announced that she will be hanging up her racquets soon.
Though the 40-year-old did not reveal the exact time, it is presumed that the US Open will be the 23-time Grand Slam champion's swansong. Serena, who will soon turn 41, is one title shy of Margaret Court's all-time record and appears set for one last shot at matching the Australian at the Flushing Meadows, where she has triumphed six times.
Serena bid an emotional farewell to Toronto recently, where she she was knocked out of the Canadian Open 2-6, 4-6 by 12th seed Belinda Bencic.
"It's been a pretty interesting 24 hours. Like I said in my article, I'm terrible at goodbyes," she said, her voice breaking. "But, goodbye," she said, then adding with a laugh "Toronto." "It's been a joy playing here all of those years."
Before she left the court she was presented with jerseys from the NBA's Toronto Raptors and NHL's Toronto Maple Leafs - and with a child's version of the same for daughter Olympia.