Argentina kicked off their quest to win their first Olympic honours for more than half a century with a 6-0 drubbing of Serbia & Montenegro.
Heinze headed in Argentina's fifth goal then set up their sixth
Manchester United's Gabriel Heinze played a starring role, scoring one and setting up another to help send his country top of the Group C standings.
Boca Juniors star Carlos Tevez bagged a brace and Cesar Delgado, Kily Gonzalez and Mauro Rosales added one apiece.
Andrija Delibasic came close for Serbia late on but he sent his drive wide.
The hosts Greece battled back from 2-0 down in their opening Group A game against South Korea to draw 2-2.
Dong Jin Kim and Loukas Vyntra put the visitors ahead and with 12 minutes remaining they appeared to be on track for a famous win.
But late goals from Ioannis Taralidis and Dimitrios Papadopoulos, with a controversial penalty, got the home side back on terms.
In the other Group A encounter, outsiders Mali held Mexico to a 0-0 draw, while Group C rivals Tunisia and Australia shared a 1-1 draw.
In the women's tournament, Germany crushed China 8-0, Birgit Prinz contributing four to the biggest winning scoreline ever recorded for the event.
Prinz's goal haul was also a record for most goals scored by one player in a women's match at the Olympics.
Greece were humbled 3-0 by a Mia Hamm-inspired United States - her 152nd goal in 261 games for her country.
Marta's lone first-half strike sealed a 1-0 win for Brazil over Australia, while Japan overcame Sweden by the same score thanks to Eriko Arawaka's 25th-minute strike.