Roger Federer has lost 4-6, 6-4, 6-3 to France's Gilles Simon at the Masters Cup in his first match since a sore back forced him out of his previous tournament.
The second-ranked Swiss star said earlier that he didn't know what to expect when he played his first round-robin match on Monday in the season-ending event for the world's top eight players.
It turned out to be flashes of his usual brilliance, then a quick slide downhill. Federer faltered late in the second set with a rash of errors that allowed Simon to get back into the match.
Ninth-ranked Simon was only added to the field when world number one Rafael Nadal withdrew with knee tendinitis.
The crowd overwhelmingly favoured Federer. Swiss flags were scattered around the nearly packed Qi Zhong stadium, and one section was a sea of red and white.
Federer also lost his first match at last year's Masters Cup but went on to win the title. He also won the Cup in 2003, 2004 and 2006.
In the other match in Federer's group, Britain's Andy Murray beat American Andy Roddick 6-4, 1-6, 6-1.