Paris - World number one Rafael Nadal overcame a second-set blip to reach the Paris Masters quarter-finals on Thursday with a 6-3, 6-7 (5/7), 6-3 victory over Uruguayan Pablo Cuevas.
The Spaniard, 31, had sealed the year-end number one spot for a fourth time on Wednesday, but he was made to work hard by an enterprising Cuevas.
Nadal is looking for his maiden title on the Bercy hard courts, which would take him clear of Novak Djokovic with a record 31st Masters triumph.
The 16-time Grand Slam champion will take on world number 77 Filip Krajinovic in the quarters, after the Serb beat Nicolas Mahut 6-2, 3-6, 6-1.
"You don't expect to win easy in big tournaments like these ones," said Nadal, who had his right knee strapped after the second set.
"It's normal that at the end of the season after all the things that happened... All the matches I played with high intensity, it's normal that there are body issues a little bit.
"So it's nothing strange. Something that's completely normal and accepted."
Nadal started far stronger than he did in Wednesday's win over Hyeon Chung, breaking early on thanks to a couple of trademark forehands.
Cuevas played his part during the first set in front of a near-capacity crowd, hitting an outrageous 'hot-dog' shot between his legs for an outright winner.
But Nadal held after a tough service game for a 4-1 lead before closing out the opener on his fourth set point.
The reigning French and US Open champion has only won two of his 75 ATP titles indoors, with just one at this level coming at the 2005 Madrid Masters.
He broke in the third game of the second set, only to hand Cuevas a lifeline with a string of uncharacteristic unforced errors as the world number 36 hit straight back.
The South American forced a tie-break, but after Nadal had moved 4-2 in front, the top seed dumped two volleys into the net and Cuevas levelled the match at the first opportunity.
Nadal returned to the court reinvigorated after having his knee taped, breaking serve in the second game with a fierce backhand down the line.
Cuevas also had to call the trainer after appearing to dislocate his thumb when sliding for the ball, but he grabbed the break back as Nadal struggled to put the match to bed.
That proved to be the unseeded player's final show of resistance, though, as his illustrious opponent reeled off three successive games before wrapping up the match with a big serve out wide.
Argentina's Juan Martin del Potro closed in on a place for the World Tour Finals after beating Robin Haase 7-5, 6-4.
The former US Open champion will take the eighth and final qualification spot for London from Pablo Carreno Busta if he reaches the semi-finals at Bercy.
He will face ninth seed John Isner in the last eight after the American beat Grigor Dimitrov 7-6 (12/10), 5-7, 7-6 (7/3).
Last year's runner-up Isner will qualify for the Tour Finals if he wins the tournament in Paris.
"It's going to be a good battle next round trying to qualify for London," said Del Potro.
"But anyways, I think I have two ways to take, one is London, one is my home and vacation."
The South American's victory means that 12-time Grand Slam winner Djokovic will drop out of the world's top 10 next week for the first time since March 2007.
Belgian David Goffin sealed the penultimate Tour Finals spot despite slumping to a 6-3, 6-3 defeat by journeyman Frenchman Julien Benneteau.
Lucas Pouille's slim chances of reaching London ended, though, as the home player lost 7-6 (8/6), 6-3 to the big-serving Jack Sock.
The American will next face veteran Spaniard Fernando Verdasco, who knocked out Austrian fifth seed Dominic Thiem with a 6-4, 6-4 victory to reach his first Masters quarter-final since 2012.
Results from the ATP Paris Masters on Thursday (x denotes seeding):
Rafael Nadal (ESP x1) bt Pablo Cuevas (URU) 6-3, 6-7 (5/7) 6-3
Juan Martin del Potro (ARG x13) bt Robin Haase (NED) 7-5, 6-4
John Isner (USA x9) bt Grigor Dimitrov (BUL x6) 7-6 (12/10), 5-7, 7-6 (7/3)
Fernando Verdasco (ESP) bt Dominic Thiem (AUT x5) 6-4, 6-4
Filip Krajinovic (SRB) bt Nicolas Mahut (FRA) 6-2, 3-6, 6-1
Julien Benneteau (FRA) bt David Goffin (BEL x7) 6-3, 6-3
Jack Sock (USA x16) bt Lucas Pouille (FRA x17) 7-6 (8/6), 6-3
Marin Cilic (CRO x3) bt Roberto Bautista Agut (ESP x 14) 7-6 (7/4), 6-2