“I’ve studied heavyweight boxing my whole life,” Fury said.

So when Fury stunned Wladimir Klitschko in November 2015 for the world heavyweight title, the victory was the fulfillment of a dream. But that was the problem: Fury said he suddenly felt as if he had nothing left to achieve. He had fame and fortune, but sensed that he was in trouble.

“I didn’t want to box again,” he said, “and I knew that straightaway.”

Still, Fury was scheduled for a rematch with Klitschko when he gave a rambling interview that landed on YouTube in May 2016 and that included homophobic, misogynistic and anti-Semitic remarks. Amid calls for him to be barred from the sport — Fury had a history of making offensive comments — he issued a public apology.

Asked this month whether he regretted having made those remarks, Fury declined to comment, saying: “Anything that’s going to get me in trouble, I’m not willing to talk about. I only want to talk about boxing.”

He added: “All of these things, they’re just my opinions, so they don’t mean anything, anyway. What does a fighter’s opinion mean on politics? Nothing. Donald Trump’s the president of the United States. It’s his opinion that means something, not mine.”

In any case, Fury was beginning to spiral. His rematch with Klitschko was delayed, then called off, when he failed two drug tests. Fury gave up his belts and had his boxing license suspended in October 2016.

“I saw the depths of Tyson’s out-of-control stage,” said Stephen Espinoza, the president for sports and event programming at Showtime, which is broadcasting the fight on pay-per-view. “At the time, I don’t think anybody thought he would ever fight again. I think most people were just wondering if he would be O.K.”