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There was a relaxed atmosphere in Inalpi Arena as the Italian had already secured top spot in the Bjorn Borg Group and a semi-final against Australia's Alex de Minaur, while Shelton had been eliminated after losing his two previous matches.

Shelton was broken in the opening and closing games of the first set, unable to take advantage of a break point at 2-1 down while Sinner was always capable of pulling out an ace at the crucial time, hitting two in that fourth game to hold serve.

The American put up more fight in the second set, serving to love on three occasions, rescuing a match point at 5-4 down and forcing Sinner into a tiebreak for the first time in the last two editions of the season-ending championships, before the Italian sealed the win.

Shelton, making his ATP Finals debut, suffered defeats to two-times winner Alexander Zverev and Canadian Felix Auger-Aliassime.

Sinner's chances of ending the year as world No.1 evaporated when Carlos Alcaraz completed a clean sweep in the Jimmy Connors Group with a win over Lorenzo Musetti, leaving little at stake against Shelton apart from his unbeaten run.

"Serving very well in important moments has brought me to this point," Sinner said.

"When you come here and win all three round-robin matches, you have to play at a very high level, which I've done.

"My mental aspect has been great until now, so let's see what's coming."

Before Sinner and Shelton emerged, Alcaraz was presented on court with the ATP year-end world No.1 trophy, which the Italian won last year, and the pair may yet do battle one last time in 2025 in the final in Turin.

"It's a pleasure being the number one of the world, it's something that I'm working really hard for every day, it is a goal to be honest," Alcaraz said.

"For me it's a great achievement. It means the world to me and I'm just really proud and happy."