Novak Djokovic described his return to the world number one ranking as a "phenomenal achievement" as he and old rival Roger Federer coasted into the Paris Masters quarter-finals.
Four-time Paris champion Djokovic made the last-eight when Bosnian opponent Damir Dzumhur retired injured trailing 6-1, 2-1 on Thursday.
Federer defeated Italy's Fabio Fognini 6-4, 6-3 as the Swiss great moved three wins away from his 100th career title.
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.
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"Reflecting on what I've been through in the last year, it's quite a phenomenal achievement," said Djokovic who will face Marin Cilic for a spot in the semi-finals.
"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."
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"In the next match against Novak it will be another big challenge. (It will be) great to play him again."