SYDNEY, Australia — Novak Djokovic, the Serbian tennis star, won a legal victory on Monday in his bid to avoid deportation from Australia, as a judge ordered the government to release him from detention and restore a visa it had canceled because Djokovic has not been vaccinated for Covid-19.

The ruling, approving an agreement between the two parties, came five days after Djokovic was detained at an airport upon arrival on a flight to Melbourne from Dubai. He was hoping to defend his title at the Australian Open, which he has won a record nine times.

Djokovic arrived with a visa and a vaccination exemption to play in the tournament, which begins Jan. 17, but border officials canceled the visa with the support of Prime Minister Scott Morrison. The authorities said that Djokovic did not qualify for an exemption from the requirement that everyone entering the country be fully vaccinated.

While Monday’s ruling seemed to open the door for him to compete, the government warned that it may cancel his visa a second time even after the court’s decision.

The drawn-out conflict over the world’s top men’s tennis player, who arrived seeking to win a record-breaking 21st Grand Slam title, landed at the start of an election year in Australia and kicked off another round of international debate over vaccine politics.

With the Omicron variant pushing Covid case numbers to new heights both in Australia and the rest of the world, Djokovic’s detention pit those who argue that vaccination is more important than ever for preventing serious illness against those who insist that no one should be forced into inoculation.

On multiple occasions, Djokovic has stated his opposition to vaccine mandates, saying that vaccination is a private and personal decision. He had not, however, revealed until last week whether he had been vaccinated.

In a court filing on Saturday, Djokovic’s lawyers said that the tennis star had tested positive for the coronavirus in mid-December, and that he had been granted a vaccination exemption by Australian tennis officials on these grounds. In court on Monday, the lawyers argued that the Australian government had erred in canceling Djokovic’s visa over the vaccine requirement.

Anthony Kelly, the federal court judge overseeing Djokovic’s appeal, scrutinized a transcript of the tennis player’s interaction with border officials at the airport, pointing out that he was “incommunicado” from 4 a.m., when he complied with an order to turn off his cellphone.

According to the judge, the authorities promised to let Djokovic speak to his team and Tennis Australia at 8:30 a.m., only to cancel his visa at 7:42 a.m.

The judge noted that Djokovic’s visa application had included the medical exemption from a physician, supported by an independent panel convened by the state government in Victoria, which includes Melbourne.

He also seemed to accept that Djokovic was confused about what had gone wrong, and eager to resolve the issue.

“The point I’m somewhat agitated about is, what more could this man have done?” Judge Kelly said.

The federal government’s lawyers said Monday that Djokovic could be denied entry if he presented a risk to public health. No visitors to Australia are guaranteed admittance upon arrival, and all are subject to further checks at the border, the government argued in court filings, adding that past Covid-19 infections were no longer a valid reason to defer immunization against the virus.

Under vaccine guidelines issued in December by the country’s chief medical body, travelers arriving in Australia are expected to be vaccinated against Covid-19 after recovering from “acute major medical illness.” The government argued that “the evidence is that the applicant has recovered.”

It is not clear if or when Djokovic was ill. On Dec. 16, the day he said he tested positive, he appeared at a live-streamed public event. The following day, he appeared at an awards ceremony for junior players, where photographs showed that he was not wearing a mask.

What is clear, even to many Australians who say that the rules should be applied to everyone, including sports superstars, is that they are embarrassed by the whole affair. Australia’s entry process for the tournament, and international travel generally during the pandemic, has been marred by confusion, dysfunction and political point-scoring that all add up to an image of incompetence.

Djokovic inadvertently joined the fray on Tuesday, when he announced on Twitter that he had received a medical exemption from the requirement that all people entering Australia be vaccinated or quarantine for 14 days upon arrival.

In a statement later that day, Craig Tiley, the chief executive of Tennis Australia, explained that players seeking an exemption had to pass muster with two panels of medical experts. The process included the redaction of personal information to ensure privacy.

Communications between national health officials and Tennis Australia, and between Tennis Australia and players, have revealed contradictory messages about whether unvaccinated people infected with the coronavirus during the past six months would receive an automatic medical exemption.

Federal officials wrote to Tiley in November to indicate that testing positive for the virus during the past six months would not be sufficient to gain automatic entry into the country without vaccination. But letters leaked to Australian news outlets showed that an adviser to Australia’s federal chief health officer had also told Tennis Australia that the state of Victoria, where the tournament is being held, was responsible for assessing exemptions.

On Dec. 2, Brett Sutton, the chief health officer in Victoria, wrote to Tennis Australia: “Anyone with a history of recent Covid-19 infection (defined as within 6 months) and who can provide appropriate evidence of this medical history, is exempt from quarantine obligations upon arrival in Victoria from overseas.”

Five days later, Tennis Australia passed on the message to players.

Djokovic landed at Tullamarine Airport in Melbourne around 11:30 p.m. on Wednesday. After a nearly 10-hour standoff at the airport, border officials said he would have to leave the country. He was held in a room overnight over the validity of his visa and questions about the evidence supporting his medical exemption.

His team filed a legal challenge to the ruling on Thursday. A judge said Djokovic would be allowed to remain in Australia at a hotel that houses refugees at least until Monday as his lawyers awaited a hearing.

By that point, the decision had already become political. Australian leaders have a long history of winning elections with tough talk on border enforcement, despite the country’s harsh treatment of asylum seekers in offshore detention centers, and Mr. Morrison has followed a predictable script.

Facing a tough re-election campaign as the economy starts to seize up from a surge of absences caused by an Omicron outbreak and a shortage of testing capacity, he pounced on the decision to cancel Djokovic’s visa, trying to frame it as a clear-cut case of law and order.

“Rules are rules,” he said, adding, “Our government has strong form when it comes to securing our borders, and I don’t think anybody doubts that.”

Critics of Australia’s immigration policies said they were dismayed, but not surprised. The hotel where Djokovic is staying holds dozens of refugees, including some who have been detained for nearly a decade.

“As a country, we have been shown over time to be very aggressive in enforcing immigration policy,” said Steven Hamilton, a former Australian Treasury official who teaches economics at George Washington University. “People overseas should view this through that prism rather than as a health measure. It has nothing to do with health.”

On Friday, border officials told the Czech doubles player Renata Voracova that she, too, would have to leave the country, even though she had played matches in tuneup tournaments last week.

Voracova, who was given a medical exemption because she has had Covid-19 during the past six months, was moved to the same hotel as Djokovic, but opted to leave the country voluntarily rather than fight the deportation ruling.