Novak Djokovic will end the year as world No 1 for a record-extending eighth year after opening his ATP Finals campaign with a 7-6, 6-7, 6-3 victory over Holger Rune in Turin on Sunday.
It has been yet another sensational season from the 32-year-old Serb, who has won six titles including the Australian Open and US Open to take his record Grand Slam trophies tally to 24.
To ensure he ended the year at the top of the rankings, Djokovic needed just one win at the season-ending tournament – or have rival Carlos Alcaraz lose one match – and he prevailed in a three-hour Green Group clash with spirited debutant Rune.
Djokovic, on a run of 19 straight wins, came into the Finals off the back of his seventh triumph at the Paris Masters, his 40th victory at a Masters 100 event.
Those are two more records to add to a long and growing list for Djokovic, who is hunting a seventh Finals title which would take him one ahead of retired great Roger Federer.
"It means a lot. You could see that there were a lot of emotions on the court," said Djokovic. "I could feel it, I was very eager to win the match and get that monkey off my back. Obviously a big objective, a big goal has been achieved, everything else now is a bonus."
Djokovic looked on course for victory when he took a brutal opening set in decisive fashion, taking the tie-break with a stunning cross-court return winner.
The match looked to have turned decisively in the third game of the second set, after Rune won the first two games, Djokovic broke back in a brilliant 11-point game to eventually win at the third attempt.
Rune kept plugging away, and after the pair slugged each other with powerful hits, Djokovic slipped in the tie-break, missing a simple volley to draw level on the second point and then collapsing to concede the set.
But after briefly losing it to allow Rune to break back in game three of the third set, as Djokovic kicked his racquets in a rage, the top seed composed himself and saw out a historic win.
"I found a way to win today against an opponent who played brilliantly... he had a lot of courage today," said Djokovic in Italian. "If I want to keep up with these youngsters I'm going to have to work even harder!"
In the opening day's first match, home hope Jannik Sinner got off to a near-perfect start by beating Stefanos Tsitsipas in straight sets, 6-4, 6-4, delighting the crowd in Italy's northwest.
Sinner has had the best season of his career to date, winning four tournaments and is set to become the first Italian ever to finish a season in the ATP top five.
On Monday, Alcaraz makes his ATP Finals debut in the Red Group against Alexander Zverev, the second seed Djokovic's closest challenger.
However, the Wimbledon champion is in uncertain form after being dumped out of the Paris Masters by Russian qualifier Roman Safiullin following return from an injury lay-off.
The Spaniard, who missed last year's tournament with injury, hadn't played since the Shanghai Masters in early October due to lower back and left foot problems.