Tennis superstar Novak Djokovic trained at the Australian Open venue Tuesday in search of a record 21st Grand Slam but his dream hung in the balance as the government pondered cancelling his visa, again. The world number one had 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 unvaccinated 34-year-old Serbian ace says he is now determined to stay in Melbourne and compete in the Australian Open, which starts in six days. "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 six days ago 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. 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. "Truth and justice came to the light. I would like to thank the justice system of Australia," said his brother Djordje. Much of Australia's media said Tuesday doubts had emerged over the accuracy of Djokovic's travel 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. But the player had reportedly been in Serbia before Spain. A spokesman for 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 fan 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," he 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 — some for many years. Hundreds of Djokovic fans, anti-vaccination protesters and migration rights activists had rallied outside the centre during his stay. On Tuesday morning, there were just two television reporters outside and no protesters. A lone cardboard sign read: "Free Novak and all the refugees". 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". Rafael Nadal, one of Djokovic's main rivals for the title, said "it is the fairest thing" for the Serbian to play in the Australian Open. "Regardless of whether or not I agree on some things with Djokovic, without any doubt, justice has spoken," Nadal told Spanish radio station Onda Cero. Tennis great Martina Navratilova said on social media: "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." Although it had no bearing on his court case, Djokovic's claim of a positive test on December 16 stirred controversy after it emerged he had attended a gathering that day for the Serbian national postal service, which launched a stamp series in his honour. And pictures shared by the Belgrade tennis federation showed him at a young players' event in the city on December 17. It was reported that he handed over cups and prizes to players. No one was wearing a mask.