?Grateful?: Novak Djokovic thanks judge and takes to tennis court after release

Novak Djokovic ended an extraordinary day in Melbourne with his Australian Open dreams revived - for now - after winning an appeal against his visa cancellation. But his battle to stay in the country may yet have another twist in the tale, with the Australian government threatening to revoke his entry visa for a second time.

The circumstances have left the the world's No 1 men's tennis player, who spent four days in an immigration detention centre after his medical exemption from strict coronavirus vaccination rules was denied by border officials, in limbo days before he is due to begin the defence of his title next Monday.

A remarkable 24 hours also saw Djokovic's fans pepper-sprayed by police as they celebrated his release in Melbourne, his mother claim that her son had been "tortured" while in custody, and his father appeal to Queen Elizabeth to put pressure on the Australian government.

However shortly after emerging from his lawyers' offices a free man when a judge quashed the decision to deny his entry to Australia, Djokovic insisted that he wanted to focus purely on his tennis in the coming days.

"I'm pleased and grateful that the judge overturned my visa cancellation," he wrote alongside a photo of himself and his team on court at the Rod Laver Arena, where he has won a record nine men's singles titles. "Despite all that has happened, I want to stay and try to compete at the Australian Open."

"I remain focused on that. I flew here to play at one of the most important events we have in front of the amazing fans."

It is unlikely to be that straightforward. Australia's immigration minister, Alex Hawke, is still considering revoking Djokovic's visa using powers granted to him by Australia's Migration Act. A spokesman for Mr Hawke said the decision was an "ongoing process" and a decision is expected in the next 48 hours.

Hawke, one of the closest political allies of prime minister Scott Morrison, now has a momentous political decision to make: let Djokovic stay and play for a record 21st grand slam singles title; or deport him, which comes with a hefty three-year ban from re-entering Australia.

To exercise that power, Hawke will have to be satisfied that there is a ground to cancel the visa - in this case the alleged threat to public health - and that it is "in the public interest to cancel the visa".

The day started with Djokovic being released after a lengthy seven-hour hearing in a Melbourne court, with Judge Anthony Kelly calling the actions of the Australian Border Force "unreasonable".

When Djokovic had arrived in Melbourne last Thursday officials had initially agreed to give him until 8.30am to speak to Tennis Australia and respond to the proposed visa cancellation after he told them he was unvaccinated and they said his medical exemption was invalid in federal law. However, his visa was then cancelled at 7.42 am and he was sent to a detention centre.

"We all play by the same rules," Judge Kelly said. "Stated in other terms: those rules were not observed."

Court documents showed Djokovic's reason for seeking a medical exemption from Australia's vaccination requirements was that he contracted Covid on 16 December.

This was accepted by Tennis Australia and the Victorian state government medical panels which granted him the exemption on 30 December. However a delegate of the home affairs minister cancelled his visa after he arrived in Australia on the basis that the tennis star might pose a risk to public health.

The Australian government argues recently having Covid by itself does not justify an exemption in federal law, because Djokovic has recovered.

His initial exemption caused a huge backlash in Australia, where 90% of the adult population are fully vaccinated, while the country imposed some of the strictest lockdowns in the world that prevented citizens from returning home.

As the news filtered through that Djokovic had been successful, supporters gathered at his lawyer's office to celebrate. Later police pepper sprayed some fans who blocked the path of a car they believed was carrying Djokovic, with some also banging on the window, throwing bottles at the vehicle and even jumping up and down on top of it.

Meanwhile in Serbia, Djokovic's mum Dijana said that the decision to revoke her son's visa was the "biggest victory" of his career. She then alleged that her son had suffered "torture" while staying at immigration hotel where refugees and asylum seekers have long complained of poor conditions.

"It is bigger than any grand slam," she added. "He hasn't broken any of their laws and he was subjected to torture, to harassment. We will hear even more about what he has gone through."

Meanwhile Djokovic's father, Srdjan, urged the queen to put pressure on the Australian authorities. "I call on the Queen of Britain, Elizabeth, the leader of the Commonwealth, to intervene and protect the human rights of my son Novak Djokovic and to stop the political prosecution that has been carried out against him since he came to Australia," he said.

"Over the past few days, it's been very, very difficult for everyone in the world who is free thinking," he added. "But he is mentally extremely, extremely strong. They took away all his rights as a human being. He refused to revoke his visa. They gave him no right to prepare his defence for several hours, and they took away his phone. Fortunately they gave him back his phone. He contacted his legal team who mounted a fantastic defence, that they could not match."

However in the days ahead Djokovic will likely also be asked to justify why he posed for photographs with children at a prize-giving ceremony in Serbia the day after a positive PCR test was confirmed.

Photographs on the Belgrade Tennis Association's Facebook page posted on the 17 December show Djokovic posing with a group of young people at a tennis awards ceremony. The French newspaper L'Equipe insist that he also posed for a photoshoot with them on 18 December.

However when his brother Djordje was asked on Monday about Djokovic's movements after his positive Covid test in December he immediately closed down the press conference.

Meanwhile the president of the Serb parliament, Ivica Dacic, said he expected Hawke to revoke Djokovic's visa on Tuesday.

"Obviously the Australian authorities will decide to deport Djokovic, who will then not be able to enter Australia for the next three years," said Dacic, a former Serbian prime minister whose Socialist Party is part of the ruling coalition.

Dacic said the incident would leave "a lasting mark on relations between Serbia and Australia. "Every state has the discretionary right to refuse or allow entry to any person, but it never happens to prominent people such as Djokovic," he said.

"That is something you rarely see. In general, [Australia's] behaviour has been shameful and disgraceful."

*Additional reporting by Milivoje Pantovic with N1 Television in Belgrade