Rafael Nadal ensured he will end the season ranked number one with a 7-5 6-3 victory over Chung Hyeong at the Paris Masters.
It is the fourth time the 31-year-old has achieved the feat having previously finished 2008, 2010 and 2013 on top.
This has been a remarkable season of renaissance for Nadal, who bounced back from wrist problems last year to win his 15th and 16th grand slam titles at the French Open and US Open.
In an on-court interview, the Spaniard said: "I'm very very happy for everything. It has been an amazing year. One year ago for sure I never dreamed about being world number one at end of the season so it's something that means a lot."
Roger Federer's withdrawal from the tournament in order to rest ahead of the ATP Finals meant Nadal needed only one win to guarantee his great rival could not catch him.
And he avoided any slip-ups against 54th-ranked Chung, although the 21-year-old did cause Nadal problems.
Nadal sat out last week's event in Basel to rest his knees after a successful hard-court season and put in a patchy performance.
After conceding an early break, he looked to have turned things around when he led 5-2 in the opening set only for Chung to level up.
The young Korean had a break point for 6-5 but Nadal's serve helped him get out of trouble and he pulled away from a tiring Chung at the end to claim victory after an hour and 48 minutes.
Nadal has, of course, enjoyed fantastic success in Paris, winning the French Open 10 times across the city at Roland Garros, but he has never won the title at Bercy.
Next he will face Uruguayan Pablo Cuevas, who ended a 10-match losing streak in round one and backed that up with a 6-7 (5/7) 7-6 (7/1) 6-2 victory over 15th seed Albert Ramos-Vinolas.
Fourth seed Alexander Zverev looked jaded in a surprise 3-6 6-2 6-2 loss to Dutchman Robin Haase in his opening match.
In the race to join Nadal and Zverev at the ATP Finals in London, there were valuable victories for Lucas Pouille and John Isner.
Pouille, the champion in Vienna last weekend, eased to a 6-3 6-4 win over Feliciano Lopez while Isner, who would need to win the title in Paris to stand a chance, was a 7-6 (7/2) 6-7 (11/13) 6-3 winner over Diego Schwartzman.
But Kevin Anderson, the man beaten by Nadal in the final of the US Open, can no longer qualify after a 5-7 6-4 7-5 defeat by Fernando Verdasco.Let's