Sleepless nights, constant thoughts of what went wrong — and how it could have been prevented.

Jannik Sinner opened up about his ongoing doping case after beating US Open runner-up Taylor Fritz 6-4, 6-4 on Sunday to claim the ATP Finals trophy in front of his home fans at the Inalpi Arena in Turin, Italy.

Sinner tested positive in two separate drug tests in March, and a decision to clear him of wrongdoing was appealed by the World Anti-Doping Agency in September. A final ruling is expected from the Court of Arbitration for Sport early next year.

Sinner's explanation was that the banned performance-enhancer entered his system unintentionally through a massage from his physiotherapist, who had used a spray containing the steroid to treat his own cut finger.

"I've kept on thinking about where we went wrong and what we could have done better," Sinner said in Italian. "I had days where I wasn't feeling great; nights when I didn't sleep well.

"But, when I put my hat on, I don't think about what's going on beyond the court," Sinner added. "I'm fortunate to have people around me who keep me stable. Thanks to them we've continued to work hard every day like there's no tomorrow, and we've even improved in this period."

Sinner's victory in Turin was the latest big achievement in a year full of them, following his first two Grand Slam titles at the Australian Open and US Open and clinching the year-end No 1 ranking.

Sinner has won 26 of his last 27 matches and ends the ATP season with eight titles and an overall record of 70-6.

"The work will never stop. We know we still have room for improvement," Sinner said. "But, it's been an incredible season and there's no better place to finish the season than here."

Sinner maintained his recent mastery over Fritz, having also beaten the American in the US Open final in September and in the group stage at the elite event for the year's top eight players last week.

By sweeping all five of his matches en route to the trophy, Sinner earned $4.8 million — the largest winner's prize on the men's tour.

Sinner became the first Italian to win the Finals, and he went one step further than last year, when he lost the championship match to Novak Djokovic, who withdrew from this year's Finals. And he did so without dropping a set — a feat last achieved by Ivan Lendl in 1986.

"I just tried to understand whatever works best for each opponent," Sinner said. "It was a very high-level tournament from my side. At times, I couldn't play better."