World number one Rafael Nadal pulled out of the ATP Finals and brought an end to his season after defeat by Belgian seventh seed David Goffin in his opening round-robin match.
Nadal said before the tournament his knee was "not perfect" but fought hard before losing 7-6 (7-5) 6-7 (4-7) 6-4.
The Spaniard, 31, described it as a "miracle" that he took Goffin to three sets over two-and-a-half hours.
"I am off and the season is finished. I am really not ready to play," he said.
"I really fought a lot during the match knowing probably it was a big chance [that it would be] the last match of the season. It's about the pain and I cannot hold with enough power to keep playing."
It is the sixth time Nadal has withdrawn from the season-ending tournament in 13 years of qualifying and he will be replaced by compatriot Pablo Carreno Busta.
Nadal is, however, guaranteed to end the year as the world number one and will now concentrate on recuperation before preparing for the new season.
"The good thing is [the injury] is nothing new," he added. "We hope to have the right rest, the right work and try to be ready for the beginning of the next season."
Grigor Dimitrov earlier beat Dominic Thiem 6-3 5-7 7-5 in the opening round-robin match in the Pete Sampras Group.
The Bulgarian sixth seed admitted he had felt "pretty nervous" after seeing off Austrian fourth seed Thiem in three dramatic sets on his ATP Finals debut.
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