Novak Djokovic has been warned by Boris Becker that his "stubbornness" could prevent him being remembered as the greatest tennis player that ever lived.
World number one Djokovic, who has not declared his COVID-19 vaccination status, seemed set to play in this month's Australian Open after he was granted a medical exemption by tournament organisers.
That was required for any player who has not been fully vaccinated. One theory that has been widely suggested is that Djokovic may have been entitled to an exemption after a positive test for COVID-19 in the past six months, although he has not confirmed he has recently had the virus.
However, Djokovic is now set to spend the weekend in a Melbourne detention hotel in which refugees and asylum seekers are also being kept, after Australian Border Force's decision to cancel his visa application.
A court hearing on Monday should provide a resolution to the saga, with Djokovic's legal team set to battle for his right to enter the country and play in the season's first grand slam, at which he would be bidding to win a record 21st men's singles major title. The Australian Open gets under way on January 17.
Becker coached Djokovic for three seasons, from 2014 to 2016, and told the Daily Mail the 34-year-old Serbian is "making a big mistake in not getting vaccinated".
The German said the decision "is one that threatens what remains of his career and his chance to cement himself as the greatest player of all time".
Becker, a six-time grand slam winner, said: "The same incredible determination which I saw win so many close matches can be a vulnerability with his stubbornness.
"It is not just about Australia. The fact is that we are living in a different world and he is going to find it very hard to live the life of a professional tennis player travelling around without the vaccination.
"Maybe one day we will get back to a more normal situation, but at 34 he does not have much time left to pursue his goals."
There have been protests on the streets of Melbourne and Belgrade, with arrests made by police in the Victorian capital, while Djokovic found some support from within the tennis fraternity on Friday, as American John Isner backed his case.
Isner wrote on Twitter: "What Novak is going through right now is not right. There’s no justification for the treatment he’s receiving. He followed the rules, was allowed to enter Australia, and now he’s being detained against his own will. This is such a shame. #IStandWithNovak".
Nick Kyrgios, who has had his run-ins with Djokovic in the past, labelled Australia's handling of the situation "really bad", while former world number one Andy Roddick also appears to be in Djokovic's corner.
Yet Australia's home affairs minister Karen Andrews rejected any idea that Djokovic was being "held captive".
Andrews told the ABC: "Can I say, firstly, that Mr Djokovic is not being held captive in Australia. He is free to leave at any time that he chooses to do so and Border Force will actually facilitate that.
"Yes, there was a visa issued – that is actually not the issue. It is the second part of that process, which is the specific entry requirements to be able to cross Australia's border and to enter Australia lawfully."
She said Djokovic was not the only tennis case that was under investigation by Australian authorities. A player and an official are reported to be under scrutiny.
"I'm aware of investigations in relation to two individuals by the Australian Border Force," Andrews said. "They're going through their processes of investigation.
"And at some time, they will brief me, but all I can absolutely assure you and the rest of Australia of is that the Australian Border Force will take absolutely the appropriate action."