Rafael Nadal pulled out of his second-round match at the Paris Masters on Wednesday because of an abdominal problem, meaning Novak Djokovic will reclaim the No. 1 ranking next week.
Nadal was returning from a right knee injury which forced him to retire from the U.S. Open semifinals but took medical advice not to play against Fernando Verdasco.
US Open champion Naomi Osaka storms into top-10 while Rafael Nadal stays No.1 in the latest rankings
"The last few days I start to feel a little bit the abdominal, especially when I was serving," Nadal said. "I was checking with the doctor and the doctor says that is recommended to not play, because if I continue the abdominal maybe can break and can be a major thing, and I really don't want that."
At last year's tournament, Nadal reached the quarterfinals but then pulled out against Serbian qualifier Filip Krajinovic. Nadal has dealt with off-and-on knee problems for years and, given his injury record, the 32-year-old Spaniard prefers to be cautious.
At the U.S. Open in early September, he dropped the opening two sets against Juan Martin del Potro before retiring. He then skipped the Asia swing to recover, missing tournaments in Beijing and Shanghai.
"It has been a tough year for me in terms of injuries so I want to avoid drastic things," Nadal said. "Maybe I can play today, but the doctor says if I want to play the tournament, I want to try to win the tournament, the abdominal with break for sure."
Nadal did not say whether he will play at the season-ending ATP Finals in London, beginning Nov. 11.
"I cannot answer. I just go day by day, as I did all my tennis career," the 17-time Grand Slam champion said. "I would love to be in London of course. But the most important thing for me is to be healthy, be healthy and have the chance to compete weeks in a row. Something that I was not able to do this year, playing only nine events and retiring in two."
Nadal is optimistic his latest injury will pass, providing he does not rush back.
"It would not be fair to say it's a real injury today but what is sure, if I continue it will be a real injury," he said. "When you come back after injuries, and you push a little bit, the body at the beginning some issues can happen."
Djokovic, who faces Damir Dzumhur in the third round, will reclaim the top ranking for the first time in two years on Monday.
Also, Roger Federer advanced to the third round after big-serving Milos Raonic retired with a right elbow injury.
Raonic injured himself during a three-set win against Jo-Wilfried Tsonga on Tuesday.
"In the middle of the second set, I overextended my elbow and it did some kind of pain," he said. "I went and I did an ultrasound and MRI, and they found some kind of a lesion in the tricep."
Federer, who won his 99th career title at the Swiss Indoors last Sunday, will face 13th-seeded Fabio Fognini.
Defending champion Jack Sock of the United States and fourth-seeded Alexander Zverev reached the third round in straight sets.
The 16th-seeded Sock saved all four break points he faced in a 6-3, 6-3 win against Frenchman Richard Gasquet, while Zverev advanced 6-4, 6-4 over American Francis Tiafoe.
Seventh-seeded Kevin Anderson, the Wimbledon runner-up, got past Nikoloz Basilashvili 6-3, 6-7 (3), 7-6 (3).
No. 8 John Isner, No. 9 Grigor Dimitrov and No. 10 Kei Nishikori also won.
Dimitrov had 13 aces in a 7-6 (10), 6-4 win against Roberto Bautista Agut and Nishikori beat Adrian Mannarino of France 7-5, 6-4. Isner had 33 aces in a 6-3, 6-7(2), 7-6 (1) against Mikhail Kukushkin, with the big-serving American saving a breakpoint in the 11th game of the third set.
Isner and Nishikori are competing with No. 5 Marin Cilic and No. 6 Dominic Thiem for the last two spots for the ATP finals. Thiem was facing Frenchman Gilles Simon later Wednesday, while No. 11 Borna Coric was playing Daniil Medvedev.