Rafael Nadal was forced to withdraw from the Paris Masters with an abdominal injury, sending Novak Djokovic to the top of the ATP rankings.
World number one Nadal had been set to face compatriot Fernando Verdasco on Wednesday, but he announced shortly before the scheduled start time that he would not be fit to play.
With Djokovic only needing to better Nadal's performance in Paris to take top spot, the Wimbledon and US Open champion will become world number one for the first time in almost two years next week.
Nadal revealed he was advised by his doctor that he would not be able to play the full ATP 1000 tournament, describing the latest setback of a "tough year".
"As everyone knows, I have been out of competition since the US Open," he told a news conference. "I have taken some time off, come back and it was great to be here in Paris.
"I've practiced with the guys and felt, in terms of tennis, better than I thought one week ago.
"But in the last few days, I've started to feel the abdominal a little bit - especially when I was serving. I checked with the doctor and the doctor said it is recommended I do not play.
"If I continued, the abdominal could maybe break and be a major thing. I really don't want that. It's been a tough year for me with injuries. I want to avoid a drastic thing.
"Maybe I could play today, but the doctor said that if I want to try to win the tournament, the abdominal will break. It would not be fair for me to go on the court knowing that it will not be possible to play the full tournament.
"Of course I am not happy, but I have to accept it and stay positive."
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