Rafael Nadal has withdrawn from the ATP Finals after being severely hampered by his knee injury during a three-set defeat to David Goffin.
Goffin beat the limping world number one 7-6 (7/5) 6-7 (4/7) 6-4 at London's O2 Arena and, after the match, Nadal announced he would be pulling out of the tournament.
It is the sixth time Nadal has withdrawn from this competition in 13 years of qualifying and he will now be replaced by fellow Spaniard Pablo Carreno Busta.
Nadal's problematic right knee had forced him out of the Paris Masters earlier this month and there was always a question mark over whether he would be fit enough to play at this event.
The 31-year-old fought hard against Goffin and saved four match points in the second set, but he was visibly wincing in the decider when he appeared to be running almost on heart alone.
"I am off. My season is finished," Nadal said less than half an hour after his match had concluded.
"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, no?
"I really fought a lot during the match but knew that there was probably a big chance to be the last match of the season. That's what it is going to be.
"It is not a decision at all, it's about the pain. I cannot hold with enough power to keep playing. I tried, but seriously (it) was a miracle to be very close in the score during the match."
Nadal's absence is a shame not only for his brilliant season, that has included two more grand slam triumphs, but also this tournament, which would now appear firmly within Roger Federer's grasp.
The focus for Nadal will now be to regain fitness for the Australian Open in January and he hopes his experience of knee injuries might smooth the recovery.
"I know what I have to do," Nadal said. "The good thing is it is nothing new.
"Everybody on my team, we have the right experience on 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."
He added he knew he was going to withdraw even during the match against Goffin.
"I knew during the match, (it) is not a decision I take five minutes ago," Nadal said.
"Even winning, even losing, I was going to pull out because I was not enjoying on court at all. It was not fun to play like this."
Goffin was also playing with strapping around his left knee and it should be said he produced some brilliant moments, particularly with his singing double-handed backhand.
The Belgian, playing here for the first time by right, will now fight it out for a place in the semi-finals with Carreno Busta, Grigor Dimitrov and Dominic Thiem.
"On paper it's the best win of my career," Goffin said. "But I saw he was struggling a bit with his movement on the court and his knee was suffering a little bit.
"It was tough even if he was not moving 100 per cent."
Earlier, Dimitrov had overcome a bout of nerves to beat Thiem and secure his own maiden victory at the ATP Finals.
Dimitrov surrendered a one-set lead and then needed two chances to serve for it in the decider before edging past the talented Thiem 6-3 5-7 7-5.
"I'm not going to lie, I was pretty nervous in my first match out here," Dimitrov said.
"It's not the same as practising on this court, you feel the weight on your shoulders, in a positive way of course. I'm just very grateful to win in that manner. It's not easy playing here for the first time."