Puerto Rico caused a huge shock with a superb 92-73 victory over the USA in the Olympic men's basketball event.
The winners, who were led by 24 points from Carlos Arroyo, outplayed the three-time gold medallists throughout.
Elsewhere, Spain's NBA star Pau Gasol hit 21 points as he helped his side to a 83-58 victory over China.
World champions Serbia & Montenegro suffered a late 83-82 loss to Argentina thanks to a last-gasp score from San Antonio Spurs' Manu Ginobili.
Puerto Rico's win lit up the tournament and places a question mark on the USA's ability to claim a fourth title in a row, although they need only finish in the top four of their six-team group to reach the quarter-finals.
It was only the third defeat for the Americans in Olympic history after losses to the Soviet Union in 1972 and 1988.
"No one's in awe," said US forward Lamar Odom. "Everyone wants to come out and play their best game against us. That's the spirit of the Olympics.
"You shouldn't feel bad for us. You should feel good for Puerto Rico."
The USA left themselves a huge task after falling 49-27 behind in the first half, with Utah Jazz guard Arroyo playing a key role.
The Americans, weakened by a few big-name absentees but still favourites for gold, did come back into the match in the second half once Allen Iverson found his range.
And when Daniel Santiago fouled out with six minutes to go, they looked like they might turn things around.
But the Puerto Rico side held on for a memorable success and the Americans, for whom Iverson and Tim Duncan had 15 points apiece, need to get back on track against Greece on Tuesday.
In the day's other games, Lithuania beat Angola 78-73, Italy held on to defeat New Zealand 71-69 while hosts Greece beat Australia 76-54.