Japan cashed in on a defensive mix-up and a controversial penalty as they stunned favourites Iran 3-0 in the Asian Cup semi-finals to move one win away from their fifth title on Monday.

Yuya Osako put Japan 1-0 up after half-time when the Iranian defence fatally stopped to remonstrate with the referee, and then stroked home the penalty awarded after a replay for Morteza Pouraliganji's accidental hand ball.

Genki Haraguchi then scored in stoppage time to complete a rout which had been wholly unexpected against a free-scoring Iran side which had banged in 12 unanswered goals en route to the semis.

It was the last hurrah for Iran's long-serving coach Carlos Queiroz, who is leaving the team after nearly eight years at the helm, a reign that has included two World Cups.

Queiroz said the "innocent mistake" that led to the opening goal, when players surrounded the referee as Japan played on and scored, "destroyed (them) emotionally".

"My players stopped and everyone was expecting the referee to take action for that incident," he said, referring to a challenge on Takumi Minamino on the edge of the box.

"That moment created an emotional breakdown for my team and after that there was only one team on the pitch."

Eleven minutes later, Minamino's cross hit Pouraliganji's arm as he slid in, but Australian referee Chris Beath blew for the penalty and stood by his decision after watching a replay.

Osako stroked home the spot-kick to give Japan a 2-0 lead with 23 minutes to play, and there was no coming back for Iran whose 43-year wait for a fourth Asian title goes on.

"My players had good preparation and showed great fighting spirit. They played as underdogs," said Japan coach Hajime Moriyasu, whose team will play Qatar or hosts the United Arab Emirates in Friday's final.

"I'm happy they showed that spirit and fight, and delivered a win for the fans back home."

Queiroz's Iran have been the form team of the Asian Cup but they lacked ideas on Monday against a calm Japanese team who seized their opportunities.

At a rocking Hazza Bin Stadium, Iran lobbed balls at target man Sardar Azmoun at every opportunity but it was Japan who looked the biggest threat in the opening exchanges.

Captain Maya Yoshida headed over and Ritsu Doan saw a shot trickle wide as the Blue Samurai were anything but cowed by the physical Iran presence.

However, Shuichi Gonda had to be sharp to keep out Azmoun from a tight angle -- after the goalkeeper had gifted Iran possession with a botched clearance.

The game was on a knife-edge but a major misjudgement from the Iranian defence tilted it decisively Japan's way 11 minutes after half-time.

While five players were protesting Hossein Kanani's innocence over his tackle on Minamino, the forward got up and crossed to Osako, whose glancing header put Japan ahead.

It was a body-blow for Iran, but worse was to come when Pouraliganji was adjudged to have handled in the box, a decision that could have gone either way.

Osako stuck it away to all but silence the Iranian support, who then had to watch as Haraguchi stole in for the third goal to make it a lopsided victory for Japan.

Japan will be buoyed by their best performance of the tournament so far, and with a 100 percent record they shape as formidable opponents in Friday's final.