No. 7 David Goffin pulled off a three-set upset over the No. 1 seed Monday, but Nadal could have worked his way back through a round-robin format. Unfortunately, he is unable to continue battling for a title in the year-end tournament.
"I knew during the match," Nadal said after the opening loss, per Stuart Fraser of The Times. "It was a decision I took during the match. Even winning, even losing, I was going to pull out."
"I'm not ready to play," he added, per Reuters (via Yahoo Sports). "I fought a lot knowing it was my last match of the season."
The Spanish star also withdrew from the Paris Masters two weeks ago prior to the quarterfinals despite winning his first two matches.
While he had battled injuries throughout his career, Nadal had returned to the top of the sport this season with two Grand Slam titles and a third finals appearance. He won six titles throughout the year, returning to the No. 1 spot in the world rankings in August for the first time since 2014.
Even with 16 career major titles, though, he has still never won this year-end event.
It also continues a negative trend of significant injuries throughout the 2017 season, with Novak Djokovic, Andy Murray and Stan Wawrinka missing large portions of the year.
No. 2 Roger Federer presumably becomes the favorite in London after beating Jack Sock in his opening match.