Argentina's Lionel Messi, finally cleared to play in the Olympics after a long-running club-versus-country conflict, inspired the defending champions to a 2-1 win over Ivory Coast yesterday in their opening game.
Ronaldinho's Brazil struggled to beat nine-man Belgium 1-0 while former winners Cameroon and Nigeria were both held to draws, as were hosts China.
Messi was given the go-ahead by Barcelona hours before kick-off, one day after the Court of Arbitration for Sport (CAS) ruled the Spanish club was not obliged to release him.
Rather than drag him all the way back to Europe, Barcelona said they had reached a deal over insurance and release for future friendly matches.
Messi put Argentina ahead from a Juan Roman Riquelme pass before a Sekou Cisse shot rose superbly at the back post to make it 1-1.
With four minutes left Messi fired in a low, diagonal drive which was parried out into the path of substitute Lautaro Acosta who slotted the ball into the unguarded goal.
The win put Argentina top of Group A after Serbia and Australia drew 1-1 in the other game.
Ronaldinho showed flashes of inspiration against Belgium but it was midfielder Hernanes who spared Brazil's blushes by striking a 78th minute winner in Group C.
The physical Belgians had Vincent Kompany and Marouane Fellaini sent off in the last half hour for second bookable offences.
"I think the referee has made the difference today," said Belgium coach Jean François de Sart.
His counterpart Dunga, already unhappy about the food and travel arrangements, complained about the pitch.
"The ball bounces around all over the place and you need two or three touches to bring it under control," he said.
Hosts China needed a late equaliser to draw 1-1 with rank outsiders New Zealand in the same group thanks to a Dong Fangzhuo header in the 88th minute after Jeremy Brockie had put New Zealand ahead early in the second half.
Nigeria were held 0-0 by the Netherlands in Group B while the United States beat Japan 1-0.
Georges Mandjeck scored in the 81st minute to give Cameroon, winners in Sydney in 2000, a point after Park Chu Young's goal had put South Korea ahead in Group D. Italy lead after beating Honduras 3-0.
Sebastian Giovinco put Italy ahead with a spectacular shot four minutes from half-time, Giuseppe Rossi doubled their lead from the penalty spot in first-half stoppage time and Robert Acquafresca converted another penalty early in the second period to seal the win.
Results
Group A: Australia vs Serbia 1-1; Ivory Coast vs Argentina 1-2.
Group B: Japan vs US 0-1; Netherlands vs Nigeria 0-0.
Group C: Brazil vs Belgium 1-0; China vs New Zealand 1-1.
Group D: Honduras vs Italy 0-3; South Korea vs Cameroon 1-1.