Australia have been stopped in their tracks by Vietnam, falling 1-0 in their second Under-23 Asian Championship outing.
Nguyen Quang Hai broke a 72-minute deadlock at Kunshan, China, on Sunday to lift the country above the Olyroos in Group D on goal difference.
After Thursday's winning start against Syria, Ante Milicic's side now sits third after two games and have it all to do against leaders Korea on Wednesday if they are to progress to the quarter-finals.
Australia dominated possession but Vietnam proved a tough nut to crack, absorbing a heap of pressure against a side somewhat lacking in cohesion.
Recent Socceroos bolter Ajdin Hrustic had Australia's best first-half chance, curling a shot just wide of the post.
But there was traffic coming the other way too, and a classy flying save from Paul Izzo stopped Quang Hai making his mark just before halftime.
After the break George Blackwood came off the bench and looked certain to have snapped the stalemate with a superb header off an expert Keanu Baccus delivery.
But no sooner had the ball gone in off the far post the offside flag was up.
The assault continued when Blackwood hit the crossbar and Joe Champness' effort was pushed away by Bui Tien Dung.
In the end, Vietnam caught the Olyroos on the break and the subsequent scramble in the box saw the ball pop free to Quang Hai.
The midfielder struck from distance, his shot taking a lucky deflection before nestling into the corner of the net.