Novak Djokovic eventually tamed Danish debutant Holger Rune with a 7-6(4), 6-7(1), 6-3 victory in his opening round robin match at the ATP Finals to seal the year-end number one ranking for a record-extending eighth time on Sunday.
The 36-year-old Serb was stretched to the limit at times with Rune producing some fireworks for the large evening crowd in Turin's cavernous Pala Alpitour.
But after an uncharacteristically poor second-set tiebreak, Djokovic made his experience count as he began his quest for a record seventh title at the year-ender.
Earlier on day one, Jannik Sinner made a dream start in front of his home crowd with a 6-4 ,6-4 defeat of Stefanos Tsitsipas in their opening Green Group clash.
Djokovic will next face Sinner on Tuesday.
The Serbian's 19th successive win ended any chance of Spain's Carlos Alcaraz overtaking him in the rankings this week.
A sensational first set had the crowd on the edge of their seats with Djokovic edging it on the tiebreak.
But the 20-year-old Rune, who had won in two of their first four meetings, was undeterred and cranked up the power to take the set to another tiebreak which he raced away with.
Djokovic broke early in the deciding set but then handed back the advantage with a poor service game, smashing two rackets during the changeover.
Rune wavered again though at 2-3 as he lost his focus and began to rant towards coach Boris Becker.
This time there was no coming back as Djokovic eased home.
"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."
World number four Sinner, who has qualified for the ATP Finals first time having played as an alternate last year, delighted the Italian fans with a superb display.
He served superbly throughout and struck his ground strokes with venom, needing just one service break in each set to wrap up a routine 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."
