Serena Williams' former trainer Rick Macci has branded Novak Djokovic's omission from the US Open as "a joke".
The three-time winner of the US Open announced on Thursday that he would not play in the event this year, having been denied entry to the United States due to vaccination rules.
It is not the first time Djokovic's non-vaccinated status has hindered his grand slam involvement, having been deported ahead of the Australian Open earlier this year.
Djokovic had hoped to be able to feature, with the US Tennis Association saying it would adhere to any policy change by the US government that allowed him to play, but no changes were announced by the CDC.
While the Serbian will be able to feature next year, Macci has hit out at the regulations and believes it is bad for the sport.
"I don't like it, it's bad for tennis. I'm not losing sleep over it but he played last year, COVID was worse," he said.
"He played the year before, COVID was a lot worse. Now it's in the rear-view mirror but he's not vaccinated, they don't let people in the country, it's a joke.
"It makes no sense whatsoever, it's the government. It makes no sense, zero. I don't think tennis has anything to do with it. It's the government, you've got to be vaccinated to come in, it's unfortunate.
"It applies to, in my opinion, not only the best player in the world but many other people that aren't vaccinated, that maybe it affected your business, visiting relatives or whatever it impacted.
"It's just front and centre because Djoker [Djokovic], who I feel will go down as the greatest player ever to hold a racket, if he's allowed to play tournaments.
"It looks like if you don't get vaccinated, who knows where this is going to go. I'm not in favour of it and I think they should have made a special exception, but if they do it for him, they've got to do it for everybody."