Novak Djokovic said "it is what it is" Thursday as he faced the prospect of missing more tournaments in the United States over his refusal to have a coronavirus shot.

The former world number one is set to miss the prestigious Indian Wells and Miami Open ATP 1000 events after US authorities extended their requirement for international visitors to be vaccinated against Covid-19.

"I saw it, but I don't think it's official yet, so when it's official we can speak about it," the Serbian 21-time Grand Slam winner said in Adelaide, where he is preparing for the Australian Open.

When reporters at a press conference cited a statement from the US Transportation Security Administration (TSA), he replied: "I mean, if it is official then it is -- what can I do? Nothing. You know my position, so it is what it is.

"I'm hoping (to play), but if I can't go, I can't go," added the 35-year-old.

The TSA has reportedly extended the ban on unvaccinated non-citizens until April, with Indian Wells running from March 6-19 and the Miami Open from March 19-April 2.

Djokovic was prevented from entering the United States to play the US Open last year over his vaccination status, having been deported from Australia in January ahead of the Australian Open for the same reason.

He was initially banned from Australia for three years after losing a high-stakes legal battle, but the requirement for visitors to show proof of vaccination has since been lifted and his ban overturned.

Speaking about the Australian experience last week, Djokovic said: "You can't forget those events. It's one of these things that sticks with you, it stays with you for I guess the rest of your life.

"It was something that I've never experienced before, and hopefully never again."

The Serb has slipped to five in the world and risks forfeiting up to 2,000 ranking points if he does not play Indian Wells and Miami.