Djokovic has also extended his sequence of consecutive winning sets to 30, passing his previous best mark of 29 from 2015. (Photo: AP)

Paris: Novak Djokovic described his return to the world number one ranking as "a phenomenal achievement" on Thursday as he recorded yet another personal landmark in reaching the Paris Masters quarter-finals.

Djokovic will replace Rafael Nadal as the world's top player on Monday after a two-year absence.

The 31-year-old's elevation is remarkable as it comes just five months after he slumped to 22 in the world -- his lowest ranking for 12 years -- when he was knocked out of the French Open in the quarter-finals.

Since then, however, he has won Wimbledon and the US Open, taking his Grand Slam tally to 14, winning 20 consecutive tour-level matches for the sixth time, and 29 of his past 30 encounters.

"Reflecting on what I've been through in the last year, it's quite a phenomenal achievement," said Djokovic after making the quarter-finals in Paris when Bosnian opponent Damir Dzumhur retired trailing 6-1, 2-1.

"I'm very, very happy and proud about it. Five months ago, it was highly improbable considering my ranking and the way I played and felt on the court.

"I'll probably be able to speak more profoundly about it when the season is done and hopefully if I get to finish as No. 1."

Victory on Thursday meant that Djokovic has also extended his sequence of consecutive winning sets to 30, passing his previous best mark of 29 from 2015.

The four-time Paris champion, who next faces Marin Cilic in the last eight on Friday, was guaranteed a return to the ranking summit when Nadal withdrew from Paris with an abdominal injury on Wednesday.

"It's not the end of the season. Rafa obviously is struggling with injuries since the US Open. But the race is still there," added Djokovic.

"Roger (Federer) is in the race. Rafa is in the race. It depends who is going to play the ATP Finals in London. So as I said, I can't be too ecstatic about it."

Cilic, the fifth seed, secured his quarter-final ticket with a 7-6 (7/5), 6-4 defeat of Grigor Dimitrov.

'We're all tired'

Cilic fired 29 winners to advance to the last eight in the French capital for the third straight year.

The win also improved the Croat's prospects of making the ATP Finals in London starting on November 11.

"I am very close to London, to securing my spot mathematically," said Cilic, who is part of the Croatia team facing France in the Davis Cup final at the end of this month.

"In the next match against Novak it will be another big challenge. (It will be) great to play him again."

Cilic, who has beaten Djokovic just twice in 17 meetings, ended reigning ATP Finals champion Dimitrov's season.

Djokovic and Cilic were joined in the quarter-finals by Karen Khachanov who saved two match points against John Isner on his way to defeating the eighth-seeded American 6-4, 6-7 (9/11), 7-6 (10/8).

The Russian will next play Alexander Zverev after the German fourth seed beat Diego Schwartzman of Argentina 6-4, 6-2.

Later Thursday, Federer faces volatile Italian Fabio Fognini for a place in the last eight.

Federer enjoyed a walkover into the third round when Milos Raonic pulled out with an injury.

So far this week, there have been five retirements and two walkovers as the toil of the long season begins to bite.

"We are all very tired," said world number 19 Schwartzman who has been on the road for more than two months. "It's not easy."

Austrian sixth seed Dominic Thiem reached the quarter-finals in Paris for the first time, seeing off in-form Borna Coric 6-7 (3/7), 6-2, 7-5.

Thiem, also in contention for the ATP Finals, next faces defending champion Jack Sock of the United States.