American Andy Roddick's hopes of Olympic gold disappeared on Wednesday when he lost 6-4, 6-4 to Fernando Gonzalez of Chile in a third round upset in the men's singles.

Gonzalez outplayed the second seed, who was the hot favourite for gold following the shock defeat of Swiss world number one Roger Federer in the second round on Tuesday.

The Chilean 16th seed quickly broke Roddick's usually fearsome serve on centre court to take the first set and broke again for 5-4 in second before holding his nerve to seal a superb victory.

Gonzalez plays either French eighth seed Sebastien Grosjean or Spain's Feliciano Lopez in the quarter-finals. Roddick, already knocked out of the doubles, will now prepare for the defence of his U.S. Open title after the Games.