Rafael Nadal announced Monday he has cut his season short because of more health setbacks.
The world No. 2, who has been sidelined with a right knee injury he sustained in the US Open semifinals, was scheduled to play in the ATP World Tour Finals, which begin Sunday in London.
"Hello everyone: I am writing these words to you as the end of the season," Nadal wrote in Spanish on Twitter. "It was a difficult year, very good at the tennis level when I was able to play and at the same time very bad in terms of injuries."
John Isner will replace Nadal in the ATP Finals field.
Nadal, 32, was in last week's Paris Masters draw, but withdrew minutes before he was due to take the court, citing an abdominal injury. He had surgery on his right ankle Monday.
Nadal, who was overtaken by Novak Djokovic as the world No. 1, finishes 2018 with five titles, including his record 11th French Open championship, and a 45-4 record.\
Because Nadal's season is over, Djokovic is assured of finishing the year atop the ATP rankings for the fifth time.
The 32-year-old Spaniard ends 2018 with a 45-4 record and five titles, including his 17th Grand Slam trophy at the French Open in June.
Despite his successes, Nadal's season was marred by health woes. In January, he suffered a hip injury that forced his to pull out of the fifth set against Marin Cilic in the Australian Open quarterfinals. Nadal skipped the rest of the spring hard-court season, including the first two Masters events of the year, at Indian Wells and Miami.
When Nadal finally returned, he dominated the clay season, winning four of five events he played. But he competed in only three events after the French Open in June.
The Associated Press contributed to this report.