Nick Kyrgios warned Saturday Novak Djokovic would be unstoppable at the Australian Open if cleared to play, predicting the world No 1 would use his predicament as fuel to win another Grand Slam title.

The Serbian superstar was set to spend the weekend in a Melbourne immigration detention facility pending an appeal after he was dramatically refused entry to Australia over his Covid-19 vaccine status.

Kyrgios has feuded with Djokovic in the past over his response to the coronavirus pandemic, but has softened his stance, saying on Saturday he was "feeling for him".

Djokovic's visa was revoked after he failed to meet the country's tough pandemic restrictions, with a court challenge to overturn his visa cancellation set to be heard on Monday.

If successful, he will be gunning for a 10th Australian Open crown and a record 21st Grand Slam title – a milestone that Spanish great Rafael Nadal is also chasing at Melbourne Park.

Fellow 20-time Grand Slam winner Roger Federer is injured and not in Australia.

"If he's allowed to play the Australian Open, I don't want any bar of him. I reckon he's going to be pissed off," Kyrgios said in Sydney ahead of a warm-up tournament before the Australian Open on January 17.

"He's going to be very determined to play well, and stick it to everyone (because) of what's going on. I think this is just all added fuel for him, in my opinion."

"We all know how good of a competitor he is," he added.

"You don't become a great champion like that without being able to overcome some adversity like this. I'm sure he's overcome a lot more challenging times than, you know, spending a couple more extra days in hotel rooms."

Kyrgios on Friday called Australia's treatment of the Serbian "really bad" and doubled down Saturday, saying it was "not really humane, is it, what's going on".

"I think like it's just, if this was a normal person they wouldn't have to deal with all this. His life's probably hard enough as it is, and I know what that's like. I just hope it all gets sorted soon."

He added that tennis needed the outspoken vaccine-sceptic 34-year-old, forecasting that the Australian Open would get record television ratings if he played.

Kyrgios has waged a running battle with Djokovic in recent years, sparked by the Serbian's ill-fated Adria Tour exhibition in 2020 as the coronavirus pandemic raged.

Last year he dismissed Djokovic as a "tool" after he issued a list of demands for quarantined players who arrived on charter flights for the Australian Open.

The Australian claimed Saturday their rift had been blown out of proportion, but was not sure if he would reach out personally to him.

"I don't know if we're on talking terms," he said.

"But that's the funny thing about me and Novak. We actually used to be friends. We used to practice at events and again, the media has divided us."