Japan overwhelmed Vietnam 4-1 in Hanoi yet despite being beaten the tournament co-hosts still go through to the knockout rounds for the first time in 47 years after United Arab Emirates beat Gulf neighbours Qatar 2-1 in group B.

It means Japan remain in Hanoi for a quarter-final next Saturday against Australia, who hammered Thailand 4-0 to finish second in group A and relieve the pressure after a poor tournament till now.

Iraq topped group A after a dull goalless draw against Oman and remain in Bangkok to meet Vietnam in their quarterfinal, also on Saturday.

Japan ensured their fifth consecutive quarterfinal appearance with a brace from Seiichiro Maki and goals from Shunsuke Nakamura and Yasuhito Endo.

"We were nervous at the outset and our opponents were aggressive. Lucky or not, we were able to catch up early in the game and it made our life easier," Japan coach Ivica Osim said.

Despite the defeat, Vietnamese coach Alfred Riedl was over the moon.

"It is a sensation for Vietnamese football," said the Austrian.

"Only very few people had expected this result, a place in the quarterfinals."

In Bangkok, Australia had a late Mark Viduka brace to thank for their passage to the last eight in their first ever Asian Cup.

The Newcastle striker scored twice in three minutes to give the Australians their much-needed win after they absorbed waves of Thai attacks.

Defender Michael Beauchamp and substitute Harry Kewell added the icing to the Socceroos' cake with the other goals, although the score didn't reflect the immense pressure Australia had to endure in a tense second half.

"We were tested at the back, they are a good side, but we managed to score some goals which is what it is about," said under-pressure Australian coach Graham Arnold.

"Things haven't gone well for us but we are looking forward to Japan."

Japan, bidding for a third straight continental title, struggled to penetrate a determined Vietnam defence early in the match before a raucous sell-out crowd of 40,000 at the My Dinh National Stadium.

And Vietnam took the lead when Nguyen Vu Phong's corner kick from the left bounced off teammate Phan Van Tai Em and then Japanese midfielder Kenta Suzuki after a goalmouth scramble in the seventh minute.

But Japan hit back five minutes later when Maki headed home a left wing cross from Celtic playmaker Nakamura.

It was the signal for a one-sided match with Endo getting the second just after the half hour mark before a trademark Nakamura left-foot drive made it three. Maki wrapped it up with a fourth in the 59th minute.

In the other group B match in Ho Chi Minh City, Faisal Khalil ruined Qatar's ambitions with a last minute winner to prevent their Gulf neighbours making the quarterfinals.

A first half penalty by Uruguay-born Sebastian Quintana Soria gave Qatar hopes but Al Sharjah striker Saeed Al Kaas brought them down to earth with a well-placed header on 59 minutes.

Faisal Khalil rubbed salt in their wounds when he pounced on a rebound from an Ismail Matar free-kick in stoppage time to consign Qatar to the bottom of the group.