Boris Becker says Novak Djokovic will be 'suffering in a bleak room' right now and has urged him to rethink his anti-vaccination stance.
Djokovic is currently in a immigration detention hotel in Melbourne after his visa to enter Australia was cancelled.
Australian Border Force say the world number one did not provide sufficient proof of a valid medical exemption, and so since he is unvaccinated he does not qualify to enter the country.
Djokovic is awaiting the verdict of an appeal, but for now his Australian Open hopes lay in tatters and his former coach Becker believes he must now rethink anti-vaccination stance to cement his legacy as the greatest player of all time.
"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 told the Daily Mail.
"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, and you have to accept it. 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.
"As someone fond of him, I know he will be suffering. He will be shocked at the treatment he is getting, in a bleak room with his meals shoved under the door.
"It will be all the more disconcerting because he loves playing in Australia and perhaps no stadium more than the Rod Laver Arena."
Djokovic's detainment in Australia has been global news, and although Becker is a staunch backer of the Serbian he admits on this occasion he believes he has got it wrong.
"On this occasion I think he is making a big mistake in not getting vaccinated," Becker told The 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."