World tennis No. 2 Rafael Nadal has pulled out of the ATP Tour finals in London due to an ankle problem that requires surgery.

The Spaniard posted on Twitter that he wouldn't be competing in the 2018 showpiece. Despite saying he is set for a spell on the sidelines, Nadal made it clear he is hopeful of being ready to play again in 2019, per BBC Sport:

"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.

"Unfortunately, I had the abdominal problem in Paris last week and, in addition, I have a free body in the ankle joint that has to be removed in the operating room today.

"It is true that we had detected it for a long time and from time to time it bothered me. However, since the problem in the abdominal muscle also prevents me from playing in London, we take advantage of the moment to remove the free body and avoid future problems. This way I hope to be in full condition for the next season."

As relayed by sports journalist Reem Abulleil, Nadal's doctor provided details on the type of surgery the 17-time Grand Slam champion will have: 

With Nadal absent, United States No. 1 John Isner will compete in the ATP Finals alongside the likes of Novak Djokovic and Roger Federer.

The injury puts an end to what has been a campaign of highs and lows for the 32-year-old, who continues to establish himself as one of the all-time greats of the sport.

Nadal won the 2018 French Open at a canter, making it an incredible 11 titles at Roland Garros. He also made it to the semi-finals at both Wimbledon and the U.S. Open; in the former he was beaten by eventual champion Djokovic, before he retired during his match with Juan Martin del Potro in the latter.

While Nadal has been able to adapt his game to ensure he's still a force at the highest level, as Ben Rothenberg of the New York Times noted, when it comes to hard courts, the Spaniard does seem to have issues:

Injury problems have blighted Nadal throughout his career, and he'll be hopeful that taking some time out to get his ankle issue sorted will allow him to get off to a positive start in 2019. The Australian Open is the first Grand Slam of the year, and Nadal has only won that event once, a decade ago.

The ATP Finals start in London on November 11, with the draw for the event scheduled to take place Monday. 

Djokovic and Federer will be favourites to be crowned as the winner of the event following Nadal's withdrawal; last year's champion, Grigor Dimitrov, failed to qualify for the eight-man tournament this time round.