Minnows Mali produced a major shock at the Olympic football tournament with a 2-0 win over hosts Greece in Group D.
Sissoko helped set up Mali's second goal against Greece
The defeat leaves Greece, who are missing most of the stars from their victorious Euro 2004 campaign, bottom of Group A with one point.
Mamadi Berthe put Mali ahead after just two minutes and Tenema Ndiaye added the second at the end of the first half.
The Greeks could find no reply after the break and even missed a penalty in injury time.
Mali, who ousted 2000 gold medallists Cameroon during the African qualifying round, now have four points from two games.
The hosts must now beat Mexico in their final group game to stand any chance of advancing past the first round.
Berthe gave Mali an early lead when he turned home Jimmy Kebe's cross from 16 metres out.
Dimitrios Papadopoulos, Greece's lone squad member from Euro 2004, had a chance to level in the 36th minute but was unable to beat goalkeeper Cheick Bathily who raced off his line to block his shot.
Ndiaye sealed the win nine minutes later after a clever dribble and pass from Momo Sissoko.
Papadopoulos had a chance for a consolation goal in injury time when he was brought down just inside the area
by a combination of Moussa Coulibaly and Adama Tamboura just inside the area but fired his spot kick over the bar.
In the other Group A match, South Korea edged past Mexico 1-0 after a goal from Kim Jung-woo in the 16th minute.
Group C leaders Argentina guaranteed their place in the quarter-finals with a 2-0 victory over Tunisia, and Australia beat Serbia-Montenegro 5-1.
Wambach on target
In the women's football tournament, Abby Wambach made it 16 goals in 17 games as the United States moved closer to regaining their Olympic title with a shaky 2-0 win over Brazil.
Outplayed for much of the match in Thessaloniki, the Americans, who triumphed at the Atlanta Games in 1996, went ahead through a Mia Hamm penalty in the 58th minute.
Wambach then made sure of the points and a quarter-final spot with a second goal 13 minutes from the end.
In the other Group G match, Australia edged Greece 1-0 and are almost certain to reach the last eight unless the hosts upset Brazil on Tuesday.
In a Group F tie at Patras, China and Mexico drew 1-1 and Nigeria beat Japan 1-0 in Group E.