Two-goal Takumi Minamino admitted he was "surprised" by Vietnam after Asian Cup favourites Japan survived a scare in winning 4-2 to open their campaign in Qatar.

The former Liverpool forward looked to have set Japan up for a comfortable afternoon when he scored early, only for Vietnam to come roaring back with two first-half goals.

Minamino scored again before Keito Nakamura restored Japan's lead on the stroke of half-time, before substitute Ayase Ueda made the game safe with a late fourth.

Japan are hotly tipped to lift a record-extending fifth Asian Cup but Minamino said their opponents will not make it easy for them over the next month.

"I've been playing against Vietnam since I was young so I know how good a team they can be," said the 28-year-old, now at Monaco.

"But it wasn't just in defence -- they were really good in attack as well. I was surprised by how good they were."

Vietnam, who are coached by Japan's 2000 Asian Cup-winning boss Philippe Troussier, equalised five minutes after Minamino's opener when Nguyen Dinh Bac glanced a looping backwards header over the Japanese defence.

Pham Tuan Hai then stuffed home a loose ball at a free kick to put the Southeast Asians in front on 33 minutes.

Japan coach Hajime Moriyasu said he was confident that his players would find a response.

"They stayed calm despite going behind because they knew we had the whole 90 minutes and injury time to get the win," he said.

"They never let their concentration drop."

Minamino led the fightback by rolling a precise first-time shot just inside the far post in the 45th minute.

Nakamura then put Japan back in front with a jaw-dropping strike from the edge of the box in first-half injury time.

"The first half was very difficult because we went behind, but I think we showed our strength as a team by coming back," said Minamino.

"The first game of a tournament is always very important so I'm happy that we got three points."

Japan and Vietnam met in the quarter-finals of the last Asian Cup in 2019, with Japan winning 1-0 before losing to Qatar in the final.

Troussier led Japan to Asian Cup glory 24 years ago in Lebanon and also took them to the last 16 of the World Cup on home soil in 2002.

The Frenchman said that there was no disgrace in Vietnam losing to Japan.

"When you see where the Japanese players are coming from, when you see their experience, when you see their bodies, it's clear that we are not boxing at the same level," he said.

"But with our means and our potential we tried our best, that's why I am so satisfied."

Moriyasu left Kaoru Mitoma out of his matchday squad as the Brighton winger continues his comeback from an ankle injury.

Indonesia play Iraq on Monday in the other opening game in Group D.