Novak Djokovic reclaimed the world No.1 ranking on Monday after a two-year absence and is assured of finishing the year there.
That's because the man he overtook, Rafael Nadal, is done for 2018.
Nadal withdrew from next week's ATP Finals citing an abdominal injury and announced he was having arthroscopic surgery on his right ankle.
Djokovic will be the ATP's year-ending No.1 for the fifth time, pulling even with Roger Federer and Jimmy Connors for the second most since the computer rankings began in 1973. Pete Sampras holds the record of six.
"Reflecting on what I've been through in the last year, it's quite a phenomenal achievement," Djokovic said.
"And, of course, I'm very, very happy and proud about it.
"Five months ago, if you told me that ... it was highly improbable at that time, considering my ranking and the way I played and felt on the court."
The 31-year-old from Serbia was No.22 as late as June after beginning the year with a 6-6 record while recovering from surgery on his right elbow.
Djokovic has since gone 43-5, including Grand Slam titles at Wimbledon and the US Open to join Sampras with 14 majors - tied third behind Federer (20) and Nadal (17).
Nadal said on Twitter on Monday that he was still bothered by the stomach muscle issue that forced him to withdraw from last week's Paris Masters and opted to have ankle surgery now in preparation for 2019.
"This way I hope to be in full condition for next season," the Spaniard tweeted.
Nadal's spot in the ATP Finals will be taken by John Isner, who has been drawn in the same group a Djokovic, Alexander Zverev and Marin Cilic.
Roger Federer, who is seeking his 100th career title, headlines the other group containing Kevin Anderson, Dominic Thiem and Kei Nishikori.
