MELBOURNE: Tennis superstar Novak Djokovic trained at the Australian Open venue Tuesday for his attempt to win a record 21st Grand Slam but his dream hung in the balance as the government pondered cancelling his visa, again.

The world number one scored a surprise courtroom victory the day before, overturning the Australian government's decision to cancel his visa on Covid-19 vaccination grounds.

But the immigration minister said he may annul Djokovic's visa once more.

The 34-year-old Serbian ace says he is determined to stay in Melbourne and compete in the Australian Open, which starts on January 17.

"I flew here to play at one of the most important events we have in front of the amazing fans," Djokovic said on Instagram Monday.

Wearing a t-shirt and shorts, he limbered up in a gym on Tuesday accompanied by coach Goran Ivanisevic before heading to centre court, AFP journalists saw.

Television cameras filmed him from helicopters as he played.

Djokovic, a nine-time Australian Open champion, jetted into the country on January 5 carrying a medical exemption from vaccination due to a positive coronavirus test on December 16 last year.

After overnight questioning at Melbourne airport, border officials decided the exemption was not valid, cancelled his visa and transferred him to a detention centre pending deportation.

"I am not vaccinated," Djokovic had told the border official, according to a transcript released by the court.

- 'Biggest victory'

He expressed bewilderment that his exemption, approved by two medical panels in Australia, was not accepted.

The limited number of foreigners allowed into Australia must be fully vaccinated or have a medical exemption.

The government insists that a recent infection does not count as an exemption.

Federal circuit court judge Anthony Kelly dramatically reversed the visa decision Monday, ordering the cancellation be "quashed", that the player be released immediately and that the government pay his legal costs.

The government had surrendered after conceding that Djokovic's airport interview was "unreasonable" because the player had not been given the promised time to respond.

It was "the biggest victory in his career, bigger than all his Grand Slams", his mother Dijana said at a press conference in Belgrade.

Doubts emerged Tuesday over the accuracy of Djokovic's visa declaration, reportedly filled out before he flew in from Spain.

A copy of his declaration showed a tick in the box to confirm he had not and would not travel in the 14 days before landing in Australia on January 5.

There appears to be evidence he then travelled from Belgrade to Spain for the New Year period.

Djokovic was pictured in Belgrade on December 25, with Serbian handball player Petar Djordjic. The photograph appeared on Djordjic's Instagram page.

Then, three days before arriving in Australia, Djokovic was pictured by local newspaper Diario Sur playing at the Puente Romano club in Marbella, where he was staying in a villa.

A spokesman for Australian Immigration Minister Alex Hawke said he was "considering whether to cancel Mr. Djokovic's visa" by using his ministerial powers. But he would not comment further for legal reasons.

As Djokovic practised in Melbourne Park, some fans said he should be allowed to play.

"I can imagine some people will be pretty angry about it," said 22-year-old Harrison Denicolo.

"All I know is he came here, and then we turned him back when he got here. So, it's kind of unfair."

Ofek Dvir Ovadia, 22, said he was excited to see Djokovic play in the Australian Open.

"He will cop a fair bit of abuse, I reckon, when he plays just from the fans in general, but I hope a few people get behind him," Ovadia said.

Until Monday, Djokovic had been held at the former Park Hotel, a five-storey detention facility, which holds about 32 migrants trapped in Australia's hardline immigration system.

'Let him play'

The ATP, which runs the men's tennis tour, said the affair leading up to the court case had been "damaging on all fronts, including for Novak's well-being and preparation for the Australian Open".

Tennis great Martina Navratilova said she believed Djokovic should be allowed to stay: "Though I disagree with not getting vaccinated; at the end of the day it seems Novak did play by the rules as they were for the exemption and was burned. Let him play."

Czech player Renata Voracova said she would demand compensation from Tennis Australia after being detained in Melbourne last week on the same visa grounds as Djokovic

Voracova left Australia after her visa was cancelled and described her experience as "the darkest dream".

She said she wanted the federation to refund several hundred euros worth of flights and hotel accommodation for her and her coach.

"I'm not thinking about tennis. I'm still waking up from the shock, I haven't processed it yet. I'm exhausted," Voracova told the Denik newspaper.