The US men's basketball team went on a 10-0 run in the fourth quarter to come from behind to beat Australia 89-79.
Although well below par in shooting, the Americans's strong defence kept alive their chances of winning gold.
Also in group B, Puerto Rico were pushed hard by Angola but hung on for an 83-80 win, and hosts Greece suffered a 98-76 defeat by Lithuania.
In group A, Spain saw off Italy 71-63, while New Zealand claimed an 90-87 win over Serbia-Montenegro.
The Americans were made to fight back from a half-time deficit, LeBron James and Shawn Marion dazzling the crowd with spectacular alley-oop dunks.
Tim Duncan led the USA with 18 points while co-captain Allen Iverson had 16 and Shane Heal led Australia with 17.
"In the first half they shot about 60%, but in the second half we defended a lot better," said US coach Larry Brown.
"We did a lot better job of keeping them in front of us."
James added: "It's kind of life and death for us right now - for our team to be successful we have to play hard defence."
In other action, Sean Marks's dunk in the final seconds capped a superb comeback and gave New Zealand a 90-87 victory over world champions Serbia and Montenegro.
Serbia had a 13-point lead early in the final period but the "Tall Blacks" went on a charge to tie the match with two minutes to play.
"I think in the last quarter we played great both on offence and defence. Hitting a last-second shot is a great feeling," said Marks.
Spain booked a place in the last eight by beating Italy 71-63.
Propelled by Memphis Grizzlies' Pau Gasol, Spain are certain to improve on their ninth place at the Sydney Olympics.
Gasol had 16 points and Jorge Garbajosa added 17, the pair combining for all but five points in a decisive 18-8 run.