Rafael Nadal was stunned 7-6 (7/5), 6-7 (4/7), 6-4 by David Goffin in his opening match at the ATP Finals on Monday in a major blow to his hopes of winning the trophy for the first time.
The Spanish world number one came into the tournament with doubts swirling over his fitness after pulling out of the Paris Masters with a knee injury, but initially showed no obvious ill-effects during the round-robin encounter.
The loss is not fatal to his hopes of triumphing at London's O2 Arena but heaps the pressure on Nadal for his remaining two group matches and makes a mouthwatering clash with his ageless rival Roger Federer less likely.
"It was a tough fight until the end," said a relieved Goffin after the encounter, which lasted more than two-and-a-half hours.
"Rafa is one of the strongest players mentally on the tour.
"I am so happy to finally find the key to win this match. It is so special to do it here.
"I don't know how I picked myself up after losing the four match points. I had no regrets and I just wanted to keep going and try to enjoy every point."
Nadal, 31, struggled to find his range early in front of a raucous crowd rooting for him and was broken twice in the first set by the Belgian seventh seed, who had lost their previous two encounters.
The Spaniard appeared to have earned a psychological edge when Goffin double-faulted while serving for the set and the match went into a tie-break but the Belgian re-found his focus to come out on top.
The second set went with serve until the eighth game, when Goffin broke and served for the match, but Nadal dug deep to earn a break point of his own and Goffin double-faulted to allow the Spaniard back into the set.
Goffin had a golden chance to seal the match in Nadal's next service game but failed to put away a mid-court forehand on match point and the left-hander levelled at 5-5.
The Belgian composed himself to hold and then earned three more match points but Nadal, showing incredible grit and playing right at the limit, saved all three to take it to another tie-break, which he won.
In the decider, Nadal appeared to feel discomfort in his right knee as he went two breaks down but still he refused to admit defeat, recovering a break before eventually succumbing.
Nadal has won 75 singles titles in his illustrious career, including 16 Grand Slams and 30 Masters events but he has yet to triumph at the ATP Finals, played on quick indoor courts rather than his favoured clay.
Earlier, also in the Pete Sampras group, Grigor Dimitrov held his nerve to beat Dominic Thiem 6-3, 5-7, 7-5.
The elegant Bulgarian had the edge for most of an enthralling contest of one-handed backhands 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 and going on to seal victory on his third match point.
"I'm not going to lie -- I was pretty nervous, my first match out here," said Dimitrov.
"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."
In the absence of Novak Djokovic and Andy Murray, all eyes are on a potential showdown in London between Nadal and Federer, who started his bid for a seventh ATP Finals title with a straight-sets victory against American Jack Sock on Sunday.
The ATP Finals feature the top eight fit male singles players and doubles teams that have accrued the most points throughout the 2017 season.