Vietnam are currently joint top of the Group B with Japan and know victory will secure an unlikely place in the last eight for the pre-tournament underdogs.

A draw or a defeat will leave Riedl's side hoping Qatar fail to beat Gulf Cup champions United Arab Emirates at the Army Stadium in Ho Chi Minh City.

Goalkeeper Duong Hong Son and defensive pair Nguyen Huy Hoang and Vu Nhu Thanh are fitness doubts ahead of the game, but Riedl has been informed the trio should be available.

Binh Doung defender Nhu Thanh was carried off 10 minutes after the break during the 1-1 draw with Qatar, while goalkeeper Duong Hong Son required treatment on several occasions following a first-half collision.

"Vietnam football is facing the most difficult match of its history," said Riedl, who has already guided his side to a 2-0 opening-day victory over the UAE and a 1-1 draw against Qatar.

"But unfortunately, we will have to play the strongest team in the group, and maybe the title favourite in the tournament. So, it is difficult to achieve something and it is a very tough match for us to win.

"In my eyes, Japan play very good football. I like their style of play with good fast passing, using the abilities of their players.

"I've not seen that from other teams in the tournament. That means it will be extremely difficult for us to achieve one point."

A draw between Japan and Vietnam and victory for Qatar would see the places in the last eight decided by goal difference, with Qatar needing to win by three goals to be certain.

"Of course, I'd like to see a draw (in Ho Chi Minh City) so that it doesn't matter what we do. I hope the UAE can play 100% and can do something against Qatar," added Riedl.

"Even if we cannot go to the quarter-finals, it is already a big achievement what we've done until now."

Japan coach Ivica Osim is not expecting an easy ride as his defending champions look to secure a place in the quarter-finals and a route to a third straight continental crown.

"Vietnam have many advantages because they are playing at home with their home crowd in a climate they are used to. They have a chance to play a good game," said Osim.

"Whether you get through by winning the group or coming in second, it doesn't matter. All you've got to do is to get through to the next round.

"It's best to focus on the game you are playing. It's not wise to do guess work. But my staff and I can do some math. We've done all the calculations we need.

"Why don't we have anyone who asks about what if Japan lose? This is football, and we have to consider various things besides the quality of the game.

"I've told our players to play a good game of our own. They are clever enough to understand what has to be considered to play the game."