Rafael Nadal will miss November's season-ending ATP Finals in London and have surgery on an ankle injury.
In late October, Nadal withdrew from the Paris Masters with an abdominal muscle injury, meaning Novak Djokovic replaced him as world number one.
With the ATP Finals to begin on 11 November, the Spaniard, 32, said he was still suffering with that injury.
He said it influenced a decision to "take advantage of the moment" and have an operation on his ankle.
American John Isner will take Nadal's place at the eight-player event in London.
In a post on social media, 17-time Grand Slam champion Nadal wrote: "It has been a complicated year, very good at the tennis level when I was able to play, and at the same time very bad as far as injuries are concerned.