Novak Djokovic defeated Holger Rune 7-6(4) 6-7(1) 6-3 in his first match at the ATP Finals to secure he year-end number one ranking for a record-extending eighth time on Sunday.
The 36-year-old Serb faced a tough challenge in the shape of Rune, who impressed the large crowd in Turin's cavernous Pala Alpitour with his ground strokes.
Despite losing the second-set tiebreaker, Djokovic made full use of his experience as he began his hunt for a record seventh title at the year-ender.
Earlier on day one, Jannik Sinner delighted his home crowd with a 6-4 6-4 defeat of Stefanos Tsitsipas in their opening Green Group clash.
Djokovic will next lock horns with Sinner on Tuesday.
The Serbian's 19th successive win ensured that Spain's Carlos Alcaraz won’t be able to overtake him in the rankings this week.
"It took everything. The first game he was firing shots and I knew it would be a tough night," Djokovic, who now boasts an 18-1 record in his opening match at the tournament, said.
"He played great and I played great in moments. It was an emotional win knowing that I would clinch the world number one."
Meanwhile, world number four Sinner, who has qualified for the ATP Finals first time having played as an alternate last year, impressed Italian fans with a superb display.
He spectacular serving and ground strokes meant that he needed just one service break in each set to wrap up a convincing victory.
"It was a long week before coming here and I was excited to finally step on court," the 22-year-old Sinner said. "I knew it would be tough, but I think I answered the questions really well.
"It's a special week, such an incredible feeling playing here with the roof closed and the crowd."