Rafael Nadal has pulled out of next week's ATP Tour Finals in London due to injury, so Novak Djokovic will end the year as world number one having returned to the top of the rankings on Monday.
French Open champion Nadal announced that an abdominal muscle problem, which prevented him from playing in the Paris Masters last week, and ankle surgery had forced him to pull out of the season-ending event.
"Unfortunately I had the abdominal problem in Paris last week and I also have an issue in the ankle joint that has to be removed in the operating room today," the 17-times Grand Slam winner wrote on Twitter.
"This way I hope to be in full condition for next season."
American John Isner will take the 32-year-old's place in London when the Finals get underway on Sunday. Kei Nishikiori has also been drafted in to replace Argentine Juan Martin del Potro who pulled out due to a knee injury.
Spaniard Nadal's withdrawal in Paris meant Djokovic returned to the top of the world rankings. The Serbian will now become the first player since ATP rankings were introduced in 1973 to end the year at number one having started it outside the top 20.
"Reflecting on what I've been through in the last year, it's quite a phenomenal achievement," said Djokovic. "And, of course, I'm very, very happy and proud about it.
"Five months ago, if you told me that, I would be — I always believe in myself, but it was highly improbable at that time considering my ranking and the way I played and felt on the court."