Rafael Nadal's unforgettable season came to a painful end when he withdrew from the ATP Finals with a knee injury after defeat in his opening round-robin match against David Goffin on Monday.
The Spaniard, already assured of ending the year as world number one having won a 10th French Open title and the US Open, was hobbling as he went down 7-6(5) 6-7(4) 6-4.
"No, I'm off," the 31-year-old Nadal said when asked if he would be fit enough to play his remaining round-robin matches against Dominic Thiem and Grigor Dimitrov.
"My season is finished. I had the commitment with the event, with the city, with myself. I tried hard.
"I did the thing that I had to do to try to be ready to play. But I am really not ready to play.
"I fought a lot during the match, but knowing that probably ...(it was going to be) the last match of the season."
Two weeks ago, Nadal had withdrawn from the Paris Masters before his quarter-final match after experiencing pain in his right knee.
He had hoped to recover in time to attempt to claim the only major title to elude him - having twice lost in the final.
There was no questioning his fighting spirit as he managed to push world number eight Goffin all the way, saving four match points in the second set, but his movement was hampered in the final set.
He will now begin the process of recovering full fitness in time for the start of next year.
"I tried, but seriously it was a miracle to be very close in the score during the match," Nadal, who missed the end of last season to fully recover from a wrist injury, said.
Nadal has been plagued by knee injuries throughout a career which has earned him 16 Grand Slam titles.
"I know what I have to do. I know all the things that happened in the past when I had these things, and I know the treatment that I had to do," he said.
"I know the periods of time that I need to work. Then if the treatment works or not, we will see.
"The good thing is (this is) nothing new. Everybody in my team, we have the right experience (to deal with) this thing. We hope to manage it well, to have the right rest, the right work, and try to be ready for the beginning of the next season."
Spain's Pablo Carreno Busta will now step in to complete Nadal's round-robin matches.
Nadal, whose 75 career titles include only one indoors, said it had been "no fun" to play on Monday and he would have withdrawn even if he had beaten Goffin.
"Win or lose I was going to pull out because I was not enjoying on court at all," he said. "Was not fun to play like this. I don't deserve (this) after this great season to spend two more days on court with this terrible feelings.
Meanwhile, Grigor Dimitrov held his nerve to launch his ATP Finals campaign with a 6-3, 5-7, 7-5 victory over Dominic Thiem on Monday.
The elegant Bulgarian had the edge for most of an enthralling contest of one-handed backhands in front of a sizeable crowd at London's O2 Arena but admitted he felt "pretty nervous" in his first appearance at the end-of-season showpiece.
The sixth seed, who has enjoyed the best season of his career, looked wobbly as he blew his chance to serve for the match at 5-4 in the decider only to hit back immediately as Thiem himself faltered, sealing victory on his third match point.
"I'm not going to lie - I was pretty nervous, my first match out here," Dimitrov said.
"I'm just very grateful to win that match, especially in that manner. It's never easy to come out here and play for the first time."
A single break was enough for Dimitrov to take the first set, with his fourth-seeded opponent only managing to land 37 percent of first serves on the zippy blue court.
In a tight second set, the 26-year-old player earned a potentially decisive break point in the 11th game but Thiem, whose service numbers improved significantly, held firm and went on to break to love in the next game to level the match.
In a topsy-turvy decider, Dimitrov engineered a break and served for the match only for Thiem to hit back, but that only brought a stay of execution for the Austrian, who served two double faults to hand the advantage back to his opponent.
In the absence of Novak Djokovic and Andy Murray, all eyes had been on a potential showdown in London between Nadal and Roger Federer, who started his bid for a seventh ATP Finals title with a straight-sets victory against American Jack Sock on Sunday.
In the other match in the Boris Becker group, rising star Alexander Zverev beat Marin Cilic in three sets.
The ATP Finals feature the top eight fit male singles players and doubles teams that have accrued the most points throughout the 2017 season.
The tournament is organised on a round-robin basis, with the best two from each group progressing to the semi-finals.