Ronaldinho scored twice as Brazil strolled to a 5-0 win over New Zealand at the Olympics on Sunday, while a delightful goal from Ezequiel Lavezzi gave Argentina a late win over stubborn Australia.
The wins left the South American pair with six points from two games in their respective groups and guaranteed both a quarter-final place with a match to spare.
African pair Cameroon and Nigeria won for the first time, Cameroon beating Honduras 1-0 thanks to a blunder by goalkeeper Kevin Hernandez and Nigeria overcoming Japan 2-1.
Argentina, with Juan Roman Riquelme controlling the midfield and Lionel Messi tormenting the Australia defence, dominated their game but failed to convert a string of chances.
They finally broke through in the 78th minute when a series of quick passes ended with Angel Di Maria floating the ball across to the far post, where Lavezzi sidefooted home a first-time shot.
New Zealand, facing Brazil with a team mainly made up of university students, were quickly in trouble when they fell behind to an Anderson goal after three minutes before Alexandro Pato headed a second just after the half hour.
Ronaldinho added a third with a free kick which found its way through the entire New Zealand defence in the 55th minute, then won and converted a penalty six minutes later.
Rafael Sobis completed the scoring in the Group C game.
Midfielder Stephane Mbia gave Cameroon their win over pointless Honduras in Group D when a weak, low shot from outside the area caught Hernandez off his guard and rolled into the net in the 74th minute.
Second-half goals from Victor Obinna and Victor Anichebe put Nigeria in control against Japan, who replied through Yohei Toyoda in the Group B game.