World men's tennis number one Rafael Nadal has withdrawn from the Paris Masters due to injury, handing the top spot in the standings to Serbia's Novak Djokovic for the first time in two years.
It completes a remarkable turnaround for Djokovic in 2018 as he started the year poorly before returning to form.
Wins at Wimbledon and the US Open sent him flying up the rankings while Nadal struggled for fitness.
Nadal has battled knee issues since withdrawing from the final grand slam of the year in New York but it was an abdominal problem which ended his hopes in Paris.
He withdrew shortly before a second round match against compatriot Fernando Verdasco.
"It was great to be here in Paris for a couple of days and practice and in terms of tennis, I think I am better than I thought one week ago," Nadal said.
"But I started to feel something the last few days in the abdominal, especially when I was serving. The doctor says it is recommended to not play.
"It has been a tough year for me. I want to avoid drastic things. The doctor says if I try to win the tournament, the abdominal will break for sure. I am not happy but I have to accept and stay positive."
Nadal could yet end the year as No.1, especially if he can perform well at the season-ending ATP finals in London next month.
But when the new rankings are released on Monday, it will be Djokovic's name at the top for the first time since October 2016.
The in-form Djokovic, who next faces Damir Dzumhur in the French capital, has lost only twice in 34 matches as well as winning the last two Masters crowns in Shanghai and Cincinnati.
While Nadal was suffering, another of his long-time rivals moved into the last 16 without firing a shot. Roger Federer progressed without playing as Canadian opponent Milos Raonic withdrew with injury.
Raonic won a three-set encounter with home hopeful Jo-Wilfried Tsonga on Tuesday but succumbed to an elbow problem.
Federer, 37, is chasing his 100th career title will now begin his Paris campaign in the third round on Thursday against either Fabio Fognini or Marton Fucsovics.
Germany's Alexander Zverev triumphed 6-4, 6-4 against American Frances Tiafoe in a topsy-turvy encounter.
There were 23 missed break points between the pair as Zverev faced seven break points behind a poor 43-per-cent first serve success rate in the opening set, where Tiafoe could not take advantage.
However, 21-year-old Zverev finally broke serve in the 10th game to claim the first set lead after Tiafoe wilted with a netted return.
Tiafoe saved one match point in the second set but after a double-fault released a second opportunity to wrap up the contest for Zverev, who entered the third round with a Tiafoe error after 1 hour and 44 minutes.
It is Zverev's first ever victory at the Paris event after losing on his debut match last year.
Japan's Kei Nishikori advanced with a 7-5, 6-4 win over Frenchman Adrian Mannarino and American John Isner progressed after claiming 6-3 6-7 (2-7) 7-6 (7-1) victory against Kazakhstan's Mikhail Kukushkin.