France's Gilles Simon shocked top seed Roger Federer 4-6, 6-4, 6-3 on Monday at the Tennis Masters Cup, leaving the former world number one's bid for a third straight title in disarray.
Federer, who withdrew from last week's Paris Masters with a back injury, was sublime in the opening set but lost his way in the next two as Simon took control.
"I didn't serve full blast, especially from the start. I don't know if that cost me the match," Federer said.
"I was still in a position to win. It was just a pity I couldn't get to the finish line.
It was Federer's 14th defeat -- and second to Simon -- in a season in which he also lost his number one ranking after four-and-a-half years at the top and saw his five-year Wimbledon streak ended by Rafael Nadal.
"I'm very happy and very exhausted," said Simon. "It was a very long match but I played my best tennis in the last two sets."
Federer was also beaten in his first round-robin match here last year, by Fernando Gonzalez, but came back to clinch his second consecutive title and fourth overall.
The 27-year-old Swiss targeted Simon's two-handed backhand as he went a break up in the first, pummelling two deep forehands and finishing with a cross-court winner, and then converted the first set point with his second ace.