Rafael Nadal will end 2017 as world number one after a second-round win over Hyeon Chung at the Paris Masters.
The Spaniard, who has risen from ninth at the start of the year via French and US Open victories, advanced 7-5 6-3 in one hour and 48 minutes.
“The most important moment of the year was the clay-court season and winning a 10th Roland Garros title here,” he told the crowd in his on-court interview.
Nadal, 31, will play Uruguay’s world number 36 Pablo Cuevas in the last 16.
The 16-time Grand Slam champion did not have it all his own way in a see-sawing first set in which he and Chung exchanged two breaks apiece before Nadal broke to love in the final game.
South Korea’s Chung, 21, held his own in the second set but again crumbled in the latter stages, surrendering to love to give up the solitary break.
Nadal is 1,460 points clear of Swiss world number two Roger Federer in the world rankings and victory over Chung ensured him an unassailable lead before the World Tour Finals in London later this month.
Nadal, who also finished 2008, 2010 and 2013 at the top of the rankings, is the oldest player to finish a campaign at world number one since they were introduced in 1973.
Andy Murray, who was 2016’s season-end number one, is third with 20-year-old Alexander Zverev fourth.
BBC tennis correspondent Russell Fuller
A shade over 12 months ago, Nadal announced a premature end to a season which had been seriously hindered by a wrist injury. It was time to turn his attention to 2017, where in his words: “I need to create pain for my opponent.”
He has done plenty of that. His second-round victory in Paris was his 66th of a season in which he has so far won six titles and contested three of the four Grand Slam finals.
His trademark forehand has been far more menacing than in recent years, but his backhand has also been very solid and his serve – especially the second serve – much improved.
Nadal’s biggest frustration has been Roger Federer. He has simply not been able to find a solution to the puzzle presented by the 36-year-old in 2017. Their four matches have all gone Federer’s way, but there may be a fifth to close the season at this month’s World Tour Finals.
The indoor hard courts there favour Federer, but there is every incentive for Nadal as it is the one major title which has so far eluded him.