Australia vowed Friday to enforce its COVID-19 vaccine border rules "rigorously" as a decision loomed on the deportation of unvaccinated tennis superstar Novak Djokovic.

The Serbian world number one is looking to secure a 10th title at the Australian Open, which gets underway on Monday -- as well as a record 21st Grand Slam.

Prime Minister Scott Morrison's government has come under fire for not taking a decision sooner on whether to eject the 34-year-old tennis ace, an avowed vaccine sceptic.

Djokovic's legal team scored a win Monday by quashing the cancellation of his visa by Melbourne airport border control over his COVID-19 vaccination status.

Since then, Immigration Minister Alex Hawke has been threatening to use his own powers to tear up Djokovic's visa for a second time.

Hawke's spokesman said mid-week that "lengthy further submissions" from Djokovic's legal team had delayed a decision.

Finance Minister Simon Birmingham insisted Friday that Australia only lets in foreign nationals who are fully COVID-19 vaccinated or have an acceptable medical exemption.

"That policy has not changed and we will continue to apply that policy rigorously," Birmingham told national broadcaster ABC.

'A farce'

The visa battle with Djokovic is politically charged in Australia, which has endured nearly two years of some of the toughest coronavirus restrictions in the world.

General elections must be called by May.

Opposition Labor Party senator Kristina Keneally said it was now 58 days since Djokovic had been granted a visa to enter Australia.

"@AlexHawkeMP needs to decide now whether Djokovic stays or goes," she said on social media.

"The Morrison Govt is just incompetent. This is a farce."

Some tennis players say Djokovic should now be allowed to play, but not all have been supportive.

World number four Stefanos Tsitsipas criticised his behaviour.

"For sure he has been playing by his own rules," Tsitsipas said in an interview with Indian broadcaster WION.

"It takes a lot of daring to do and (is) putting a Grand Slam at risk... I don't think many players would do that."

Nearly everyone in the Australian Open had been vaccinated, Tsitsipas said. But others "chose to follow their own way which kind of makes the majority look like they're all fools".

'Misinformation'

Djokovic flew into Melbourne airport on January 5 claiming a vaccine exemption because of a positive PCR test result on December 16.

Border agents rejected his exemption, saying a recent infection was an insufficient justification, tore up his visa and placed him in a detention centre.

But Djokovic overturned the visa decision because border officials at the airport failed to give him the agreed time to respond.

As the Omicron variant races through Australia's population, Djokovic's actions have come under greater scrutiny.

The tennis ace described reports about post-infection outings without a mask in Serbia as "misinformation" in an Instagram post Wednesday.

On the day of his claimed positive test in Serbia, he attended a ceremony to honour him with stamps bearing his image. The following day he attended a youth tennis event. He appeared at both apparently without a mask.

Djokovic said he only received the PCR test result after going to the children's tennis event on December 17.

'Error of judgement'

But he admitted that he also went ahead with an interview with French sports daily L'Equipe on December 18.

"On reflection, this was an error of judgement and I accept that I should have rescheduled this commitment," Djokovic said.

The journalist who carried out the L'Equipe interview, Franck Ramella, said he had been unaware at the time of the interview that Djokovic was COVID-positive.

The tennis star also admitted to a mistake on his Australian travel declaration, in which a box was ticked indicating that he had not, or would not, travel in the 14 days before flying to Melbourne.

In fact, social media posts and reports show he flew from Serbia to Spain during that period.

Djokovic blamed his support team for this. "My agent sincerely apologises for the administrative mistake in ticking the incorrect box," he said.

Leading immigration lawyer Christopher Levingston said the immigration minister could cancel Djokovic's visa because the travel declaration was incorrectly completed.

Failure to self-isolate

But the minister may also act if he believes Djokovic may flout Australian public health orders, based on his failure to self-isolate in Serbia, he said.

Various options to appeal would be open for both Djokovic and the government, but at the end of the day, the immigration minister can exercise his personal power to cancel the visa, the lawyer said.

As COVID-related hospitalizations rise in Melbourne, the Victorian state government said Thursday it would cap capacity at the Australian Open at 50 percent.

Spectators must be vaccinated or have a medical exemption. Face masks will also be mandatory at the opening Grand Slam of the year except when eating or drinking, and those watching must socially distance while indoors.