The Serb is currently being held in isolation at the Park Hotel in Carlton, Melbourne, awaiting the outcome of an appeal against the decision by the Australian Border Force (ABF) to cancel the reigning Australian Open champion's entry visa and deport him. The appeal is set to be heard on Monday.

Djokovic had never revealed whether he is vaccinated against Covid-19, but has criticised mandates ruling that players must be double-jabbed.

The world No 1 posted on social media before setting off for the Australian Open to say he had received "exemption permission" to enter the country.

Becker - himself a former world No 1 and twice Australian Open champion, as well as winning three Wimbledon singles titles - enjoyed a successful three-year partnership with Djokovic, which included six Grand Slam victories.

The 54-year-old maintains a close relationship with the Serb, but feels their views on how to best protect against coronavirus are very different.

"On this occasion, I think he is making a big mistake in not getting vaccinated," Becker said in the Daily Mail. "It is one that threatens what remains of his career and his chance to cement himself as the greatest player of all time.

"Four times I sat in his box as he won the Australian Open, so I am fully aware of his great strengths as an incredible competitor. I also think he has a great character that can easily be misunderstood.

"Yet these strengths can also be weaknesses. The same incredible determination which I saw win so many close matches can be a vulnerability with his stubbornness."

Becker feels if Djokovic does maintain his vaccine hesitancy, it could present more hurdles within his professional tennis career.

"He is incredibly strong-willed, with very firm beliefs. If he does not, then in 10 years he will look back on it and realise he made a mistake," Becker said.

"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.

"Those are the rules, whether one likes them or not."

Nadal: Djokovic knew risks | Parents: They're crucifying him

Djokovic has been criticised by Rafael Nadal for "not following the rules", while the Serb's parents say he is a "scapegoat" and his treatment is a "political agenda".

Nadal said: "If he wanted, he would be playing without a problem.

"He has taken his own position and everybody is free to take their position. But there are consequences. I don't like the situation. In some ways, I feel sorry for him.

"But he knew the conditions months ago. He made his own decision."

On Thursday, Djokovic's father demonstrated outside the National Assembly buildings, and said of his son: "He met all the required conditions for the entry and participation at the tournament that he would have certainly won, since it's Novak, the best tennis player and sportsman in the world."

Srdjan Djokovic added: "Jesus was crucified and endured many things, but is still alive among us. Novak is also crucified... he will endure."

Kyrgios: Do better Aus | Home Affairs: Djokovic free to leave

Australia's Nick Kyrgios has since weighed in on Twitter to call for his country to "do better" in its treatment of Djokovic.

The world number 93 wrote: "Look I definitely believe in taking action, I got vaccinated because of others and for my mum's health, but how we are handling Novak's situation is bad, really bad.

"Like these memes, headlines, this is one of our great champions but at the end of the day, he is human. Do better."

Meanwhile, Australia Home Affairs Minister Karen Andrews maintained Djokovic was not being detained under duress in the country.

"[He] is not being held captive in Australia," she told ABC.

"He is free to leave at any time that he chooses to do so and Border Force will actually facilitate that."

Djokovic's Aussie Open debacle: What's happened?

Djokovic flew to Australia with a 'vaccine exemption' and arrived in Melbourne on Wednesday, but was ultimately denied entry into the country after nine hours at the airport.

The Serb's visa was one which did not allow for medical exemptions and was cancelled, after which he was moved to hotel quarantine as his team launched an appeal - this appeal has been adjourned until Monday.

What are Australia's current rules of entry?

Australia currently requires all foreign visitors entering the country to be double vaccinated, and to hold a valid and appropriate visa.

There are medical exemptions whereby some travellers may enter the country unvaccinated, but all of these are viewed and assessed alongside visas by Australia Border Force upon the traveller's arrival to immigration.

Australia Prime Minister Scott Morrison also confirmed that Australia Border Force outlined expectations of the federal government to Tennis Australia in letters sent in November regarding vaccinated and unvaccinated entry, and that a recent Covid infection was not a valid reason.

Criteria listed by the Australian Technical Advisory Group as permissible reasons for a medical exemption range from acute major medical conditions to any serious adverse event attributed to a previous dose of Covid-19 vaccine.