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.
“It was impossible for me to think about when you are coming back from a tough time, without playing tennis and with so many injuries,” he said.
Nadal, 31, will play Uruguay’s world number 36 Pablo Cuevas in the last 16.
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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.
It seemed an unlikely scenario at the end of last season when the left-hander withdrew from the French Open and missed Wimbledon with a wrist injury that then brought his season to an early end in October
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.
The race for London
Murray, along with three-time Grand Slam champion Stan Wawrinka and former world number one Novak Djokovic, will miss the World Tour Finals through injury, but the two remaining spots in the event’s eight-strong field are close to being filled.
The race for London
1. Rafael Nadal QUALIFIED
2. Roger Federer QUALIFIED
3. Alexander Zverev QUALIFIED
4. Dominic Thiem QUALIFIED
5. Marin Cilic QUALIFIED
6. Grigor Dimitrov QUALIFIED
Still in contention:
David Goffin Will qualify if makes Paris quarters
Pablo Carreno Busta Lost to Mahut in Paris
Juan Martin del Potro Will pass Carreno Busta by making last four
Lucas Pouille Must make Paris final to stand chance
French world number 15 Jo-Wilfried Tsonga dropped out of contention with a second-round defeat by compatriot while US Open runner-up Kevin Anderson’s chances ended with a 5-7 6-4 7-5 reverse to Fernando Verdasco.
Belgium’s David Goffin is a match away from confirming his place as the seventh entrant into the season finale after beating Adrian Mannarino in three sets.
He plays Julien Benneteau for a place in the quarter finals next.
However Spain’s Pablo Carreno Busta, who lost to Nicolas Mahut, could be overtaken by Juan Martin del Potro.
The Argentine, who needs to make the semi-finals, beat Portugal’s Joao Sousa 6-2 6-2.
France’s Lucas Pouille – the only other player still in the running – kept alive his faint chances alive with a 6-3 6-4 win over Spaniard Feliciano Lopez.