Paris - Juan Martin del Potro is relishing the "battle" to qualify for the World Tour Finals after beating Robin Haase 7-5, 6-4 in the Paris Masters last 16 on Thursday.
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) in a gruelling encounter.
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, who is looking for a first appearance at the year-ending eight-man event since 2013.
"But anyways, I think I have two ways to take, one is London, one is my home and vacation.
"Both ways are great to me, but of course, I would love to reach London once again."
The 13th-seeded Argentinian had little trouble in seeing off Dutchman Haase.
Three successive breaks of serve left Del Potro serving for the first set, which he took on his fourth set point.
The 29-year-old grabbed the crucial break in game five of the second set, before closing out a comfortable victory.
Del Potro, who was 47th in the 'Race to London' standings before the US Open, is now one win away from reaching his fifth semi-final in six events.
The world number 17 has suffered from a long list of injuries since he broke through with US Open glory eight years ago and was ranked 590th at the end of 2015, but he now stands on the brink of a return to the world's top 10.
"It could be a fantastic moment for me," he added.
"I didn't expect to get this close to qualifying for London.
"And everything is very amazing after all my problems, all my injuries... I love to surprise myself, and that's what I'm doing now."
The South American's win means that 12-time Grand Slam winner Novak Djokovic will drop out of the world's top 10 next week for the first time since March 2007.
Isner had to find great resolve to battle past Bulgarian Dimitrov.
The world number 14 clinched an opening-set tie-break on his sixth set point, before Dimitrov rallied to force a decider.
But Isner broke Dimitrov when the sixth seed was serving for the match with a scorching return winner, and continued his momentum to finally finish the match in a third-set breaker after almost three hours of play.
Belgian David Goffin sealed the penultimate Tour Finals spot despite slumping to a 6-3, 6-3 defeat by journeyman Frenchman Julien Benneteau.
The 35-year-old home hope claimed his biggest win since slipping from a career-high of 25th in the world three years ago to outside the top 500 in 2015.
The now 83rd-ranked Benneteau will play world number 77 Filip Krajinovic next, after the Serb saw off Nicolas Mahut 6-2, 3-6, 6-1.
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 16th seed moved into his second straight Paris quarter-final, where he will play Fernando Verdasco.
Veteran Spaniard Verdasco knocked out Austrian fifth seed Dominic Thiem with a 6-4, 6-4 victory to reach his first Masters quarter-final since 2012.
Spanish great Rafael Nadal takes on Uruguayan Pablo Cuevas later on Thursday, having already secured the year-end world number one ranking.
Results from the ATP Paris Masters on Thursday (x denotes seeding):
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