Roger Federer got his quest for a fifth Masters Cup title off to the worst possible start with a 4-6, 6-4, 6-3 defeat at the hands of Frenchman Gilles Simon on Monday.
The Swiss top seed, who withdrew from the Paris Masters 11 days ago with a back injury, made 50 unforced errors to fall to a second defeat in two encounters with the 23-year-old world number nine.
"The better you play, the better he plays, he's quite an unusual player and he makes you work hard and runs very well," said the world number two, who also lost to Simon at the Toronto Masters in July.
"Definitely today I missed shots I normally don't miss. I think that's just lack of practice.
"It's the only tournament where I really have a chance after losing the first match so I'm happy it happened here."
Third seed Andy Murray blew hot and cold on his Masters Cup debut, prevailing 6-4, 1-6, 6-1 over Andy Roddick in the other match in the Red Group, from which two players will qualify for Saturday's semi-finals.
The 21-year-old Briton rattled off three aces in his first service game on his way to wrapping up the first set but effectively forfeited the second to his American rival with a series of unforced errors.
The deciding set was equally one-sided and Murray took just 25 minutes to despatch the fifth seed, clinching victory with a leaping backhand volley.
"I'm tired," Murray said. "I felt like I played well. You know the legs don't quite have the snap in them that they did a few months ago.
"I hit the ball well for most of the match, I just didn't serve well in the second set."
Federer lost his only previous Masters Cup group match to Chile's Fernando Gonzalez in his opening encounter last year but still managed to claim the title for the second season in a row.
The 27-year-old said he had been happy with the way his back felt although he admitted he had been cautious, particularly on his serve, as he did not know how it it would hold up.
"At least now I have three sets under my belt and hopefully I'll play better in the next match," he said.
Simon got his ticket for the season finale when world number one Rafael Nadal withdrew injured and said he had gone out with the aim of taking full advantage of his good fortune.
"It was once again one of the best victories of my career," he said. "I'm really happy to be here ... I just wanted to give everything on the court."
Federer's service looked creaky in the opening set but he broke for 3-2 and looked to be cruising to a comfortable victory when he grabbed an early break in the second.
By now, though, Simon was bouncing around the court with zest. He broke back immediately before levelling the match up with a typically athletic run to the net to make a backhand volley.
Federer had already come back from 0-40 down to save his serve once in the third set when Simon got the better of him to go 5-3 up and he sealed the upset with an ace on his next service game.
"I defeated him once in Toronto, so it was easier to finish the match," said Simon. "I had no complex when I served in the last game."