Rafael Nadal pulled out of the Paris Masters on Wednesday guaranteeing Novak Djokovic's return to world No 1 spot.
Announcing his withdrawal Nadal said: "My abdominal muscles haven't felt good for the past few days, especially on service, doctors recommended that I didn't play."
Djokovic arrived in Paris knowing that he would regain top spot he relinquished two years ago if he bettered the Spaniard's performance at the indoor event he has won four times.
The Wimbledon and US Open champion becomes the first player to start the season outside the top 20 and end up top of the ATP rankings since Marat Safin in 2000.
Nadal has not played since retiring hurt in the semi-finals of the US Open against Juan Martin del Potro, where he was hit by a knee injury.
The problem sidelined him for Spain's Davis Cup semi-final against France, and also from tournaments in Beijing and Shanghai.
His withdrawal could put his participation at the season-ending ATP Tour Finals in London starting November 11 in doubt.
"It's been a difficult year for me in terms of injuries, and so I'd prefer to avoid doing anything drastic," Nadal told the media minutes before his intended match against compatriot Fernando Verdasco.
"It wouldn't have been good for me nor anyone to return to the court knowing that I wouldn't be able to play my best."
Verdasco, will play Jack Sock in the next round after the American beat France's Richard Gasquet 6-3, 6-3.
Paris Masters preview: Roger Federer takes on Milos Raonic
Paris Masters: Novak Djokovic begins No 1 bid with win over Joao Sousa as Roger Federer confirms return
Dominic Thiem, Hyeon Chung and Karen Khachanov join Mubadala World Tennis Championship
Novak Djokovic and Venus Williams confirmed for Mubadala World Tennis Championship
Nadal, 32, has enjoyed a successful year, clinching his 17th grand slam title by winning an 11th French Open in June, after triumphs in Monte Carlo, Barcelona and Rome.
But it is Djokovic who will be at the summit when the latest ATP rankings appear on Monday, irrespective of his fate in the French capital this week.
Canada's Milos Raonic has also pulled out of his second round match against Roger Federer with a right-elbow injury.
Federer has returned to Bercy event after a three-year absence, chasing his 100th career title after claiming the Swiss Indoors crown last week.
Kei Nishikori had a straightforward 7-5, 6-4 victory over Adrian Mannarino and John Isner served up 33 aces as he beat Mikhail Kukushkin 6-3, 6-7, 7-6.
Alexander Zverev of Germany beat Frenchman Frances Tiafoe 6-4, 6-4 while ninth seed Grigor Dimitrov is also through after beating Roberto Bautista Agut 7-6, 6-4.