Boris Becker admitted life in prison was "very dangerous" and could be "very lonely".
The tennis legend - who was freed from jail in December - under an early release scheme after serving only eight months of his two-and-a-half year sentence for breaching bankruptcy rules can't understand why people think being behind bars is "easy", though he acknowledged what little he knew of incarceration from TV shows and movies proved to be very different from the actuality.
He told Telegraph magazine: "It’s a proper punishment. Whoever says that prison life is easy is a liar. You have to deal with your own demons, especially in the first weeks. So you have to discipline yourself, discipline your mind, and discipline your time. If you don’t, it’s a very lonely place...
"I watched those movies, I thought I knew prison life a little. But after a week, I realised I knew nothing about it. It has its own rules, its own world, its own difficulties. And you think it’s safe, because you’re in prison, right? But prison life is very dangerous. And that’s all I’m going to tell you about that."
The 55-year-old former sportsman insisted he had little time to question whether prison had been a "humbling" experience.
He said: " 'Humbling' is one word for it. What it is, is your reality. You don’t have time to question whether it’s 'humbling' or not. It’s do or die, literally."
Boris praised his partner Lilian de Carvalho Monteiro for her support after he told her he would understand if she left him, but instead the 32-year-old beauty was a regular visitor.
He said: "Thankfully I have this incredible partner. Through thick and thin she was really there all the time."
Boris felt his daughter Anna, 22, and 13-year-old son Amadeus should stay away from the prison, but his elder sons Noah, 29, and Elias, 23, were regular visitors.
In addition, "a couple of famous friends" such as Liverpool FC boss Jurgen Klopp wanted to visit but were banned by security.
Boris explained: "Because you’re in a room full of 'proper' criminals."
The German star was also cheered by regular letters from fans.
He said: "True. I received a lot of fan mail, daily. But not every prison warden liked the fact he had to carry another 20 letters to my cell. 'You again, with your 25 letters! Who do you think you are?'
"Most of them were positive. Very stimulating, very supportive. The longer the letter, the slower I read, because then another hour’s gone."
At HMP Huntercombe in Oxfordshire, Boris gained a job as a personal trainer, where he gained valuable advice about Stoicism from the head of the gym.
He reflected: "One of the things you must learn inside, and quickly, is the word acceptance. You have to accept your verdict, accept your time, accept where you are. The only thing you can be the master of is your mind.
"I managed. I think my tennis life helped. I was probably a Stoic when I was playing, without knowing it.
"Now, I wasn’t dealing with normal students, I was dealing with criminals: murderers, rapists, people smugglers. Not an easy bunch. So you’d better be very authentic and credible… otherwise you could be in for a whooping."
