Holders Japan stormed into the Asian Cup football quarter-finals with a ruthless 4-1 demolition of co-hosts Vietnam on Monday.
But Vietnam, appearing in the tournament for the first time in 47 years, joined Japan in the last eight after Qatar lost 2-1 to United Arab Emirates in the other Group B game.
"I'm still not satisfied," Japan coach Ivica Osim told reporters. "If I was satisfied then I might as well just quit as coach."
A Keita Suzuki own goal after just seven minutes triggered riotous celebration among a crowd of 40,000 in Hanoi but it took Japan just five minutes to restore normal service.
Celtic midfielder Shunsuke Nakamura found space on the left to float over a perfect cross for Seiichiro Maki, who had the simple task of chesting the ball into an open goal.
Yasuhito Endo put Japan ahead just after the half hour, rifling a free kick into the top corner from the edge of the box to briefly silence Vietnam's trumpet-blowing fans.
Nakamura added a third seven minutes into the second half with a superb right-foot strike before Maki headed in his second from close range in the 59th minute.
"That was a good time to score," Maki told Reuters. "Shunsuke's pass over was so good it was no room for me to miss it. You feel a responsibility to score as a striker."
When news of UAE's win over Qatar in Ho Chi Minh filtered through, Hanoi's My Dinh stadium erupted in a deafening roar as Vietnamese fans celebrated wildly.
"It's a sensation," Vietnam coach Alfred Riedl told reporters. "Nobody expected us to be in the quarter-finals. It's the first time Vietnam's history.
"I'm extremely proud of my players and maybe Vietnam will surprise again."
Japan, bidding for a third straight title, finished top of Group B on seven points with Vietnam runners-up on four. UAE were third on three points with Qatar on two.
The Asian champions play the runners-up in Group A in the quarter-finals in Hanoi while Vietnam travel to Bangkok to face the winners.
The final Group A games were taking place later on Monday in the Thai capital.
