World number one Roger Federer suffered a shock 3-6, 7-6, 7-5 defeat by Chile's Fernando Gonzalez in his opening match at Tennis Masters Cup on Monday.
The Swiss, chasing his fourth title at the season-ending tournament, had never previously been beaten in round-robin play at the Masters Cup.
"I thought it was ridiculous some of the shots he was coming up with," Federer told reporters, baseball cap pulled down over his eyes.
"I can't do much when he drills that incredible forehand in the corner. I wish I had an excuse but I just lost the tiebreak in a bad way and never got the edge in the third set."
Federer had appeared to be in control after taking the opening set, but he lost his cool after Gonzalez ripped through the second-set tiebreak 7-1.
The 12-times Grand Slam champion vented his fury at the umpire on returning to his chair and his mood darkened further in a torrid third set.
Federer's frustration grew as Gonzalez valiantly saved five break-points on his way to his first victory in 11 attempts over the world's top player.
American Andy Roddick survived a scare against Russia's Nikolay Davydenko to win the earlier Red Group match 6-3, 4-6, 6-2.