Roger Federer offered no excuses for his shock defeat by David Goffin in the last four of the ATP Finals.
The Swiss superstar seemed a near certainty to claim a seventh end-of-season crown due to the absences of Andy Murray and Novak Djokovic, and the early withdrawal of Rafael Nadal, as well as his own stunning form this year.
Federer had only lost four matches in 2017 and was facing a man who had taken just two sets off him in their previous six meetings.
But defeat number five - Goffin coming from a set down to triumph 2-6 6-3 6-4 - denied Federer the chance to register an eighth title of the year, and a 96th career victory.
Goffin revealed before the match that he had come up with a different game-plan following those six previous defeats.
And Federer said: "Yeah. He played better. That was a good plan.
"It's a bit disappointing for me because it's indoors, it's a court I like to play on. But I had my chances and missed them.
"When he had them, he was very committed. I think that was the difference.
"He's crushed me too many times in practice to not also do it once in a match situation."
It appeared to be business as usual at the O2 for the world number two as he breezed through the first set in just 33 minutes.
Indeed, this had all the makings of a mismatch from the moment Federer converted a fifth break point in the first game.
Another break came in the fifth and the 19-time grand slam winner ominously served out the opening set to love with a thunderous ace.
However, Goffin broke early in the second and proved it was no fluke by comfortably taking the set as hitherto unseen errors suddenly crept into Federer's game.
In the decider Goffin secured a break in the third game which he never looked like relinquishing, with back-to-back aces settling the nerves as he served out to secure victory.
The Belgian, seeded seventh, became only the sixth player to beat both Nadal and Federer in the same tournament.
"It is the best win of my career, for sure," he said.
"Both are really special. It was the first time I'd won against Rafa. Then the semi-final for the first time here, and to beat Roger for the first time here in such a big tournament, it was the perfect moment."