There was a relaxed atmosphere in the Inalpi Arena as the Italian had already secured top spot in the Bjorn Borg Group and a semi-final against Alex de Minaur on Saturday, 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.
"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."
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.
"When he comes up with a huge serve, you cannot do a lot, so you have to accept it," Sinner said.
"My mental aspect has been great until now, so let's see what's coming."
Shelton, making his ATP Finals debut, suffered defeats to two-time winner Alexander Zverev and Canadian Felix Auger-Aliassime, who meet later on Friday in a winner-takes-all clash, with a place in the semi-finals against Carlos Alcaraz up for grabs.
Sinner's chances of ending the year as world number one evaporated on Thursday 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.
Before Sinner and Shelton emerged, Alcaraz was presented on court with the ATP year-end world number one trophy, which the Italian won last year, and the pair may yet do battle one last time in 2025 in Sunday's final.
"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."
