Rafael Nadal has withdrawn from the Paris Masters with an abdominal injury.
It means Novak Djokovic, 31, will replace him as world number one when the rankings are released on Monday, according to BBC.
The Serb will lead the rankings for the first time in two years, with Briton Andy Murray having taken top spot from him in Paris in November 2016.
Earlier on Wednesday, world number three Roger Federer made it through to the last 16 without playing when Milos Raonic pulled out with an elbow injury.
Djokovic, a 14-time Grand Slam champion, was ranked 21st going into this year's Wimbledon and will become the first player to be outside the world's top 20 and the world number one in the same season since Russia's Marat Safin in 2000.
Spaniard Nadal, 32, was due to make his comeback after nearly two months out with a knee injury that led to him retiring from his US Open semi-final.
"I felt better than I thought I would one week ago but in the last few days I started to feel a bit in my abdominal, especially when I was serving," the French Open champion said.
"I checked with the doctor and the doctor recommended that I did not play.
"It has been a tough year for me in terms of injury so I want to avoid drastic things.
"I have to think longer term. I want to keep playing tennis for a couple of years, so I have to do the logical thing."
The Spaniard has not made any decisions yet about whether he will play at the season-ending ATP Finals in London in November.
Nadal's scheduled Paris opponent, 34-year-old Fernando Verdasco, will play Jack Sock in the next round after the American beat France's Richard Gasquet 6-3 6-3.
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