Sleepless nights. Constant thoughts of what went wrong - and how it could have been prevented.

Jannik Sinner has opened up about his ongoing doping case after beating Taylor Fritz 6-4 6-4 in Sunday's final in Turin to claim the ATP Finals trophy before home fans, cementing his status as the best player in men's tennis.

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.

He 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 U.S. Open final in September and in the group stage this week at the elite event for the year's top eight players.

By sweeping all five of his matches en route to the trophy, Sinner earned $US4.8 million - the largest winner's prize on the men's tour.

He went one step further than last year, when he lost the championship match to Novak Djokovic, who withdrew this time. And he did so without dropping a set - which was last accomplished 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."

The crowd was decked out in orange - Sinner's theme color. They broke into a customary chant of "Ole, Ole, Ole; Sin-ner, Sin-ner" when Sinner produced a drop-shot winner to break for a 4-3 lead in the first set.

Sinner faced a break point while serving for the first set but saved it with a big serve out wide that Fritz couldn't return. Then he served an ace ? his 10th of the set ? to close it out.

Another break by Sinner early in the second and the match was virtually over.

Sinner served better than he had all week, leading 14-8 in aces without any double-faults.

"He served absolutely lights out. So many lines," Fritz said. "He placed the serve great. He took a lot of risk on the second serve as well. I think that was probably one of his main game plans, to not let me attack his second serve."

US Open finalist Fritz will rise to a career-high No. 4 in the rankings on Monday, making him the highest-ranked American man since Andy Roddick was No. 4 in August 2007.

Both Sinner and Fritz will conclude their seasons representing their countries in the Davis Cup finals, which start on Tuesday in Malaga, Spain, and which will also mark Rafael Nadal's retirement.