Shizuoka - Japan's talismanic leader Michael Leitch praised a "complete performance" from the home nation after they floored Ireland in a jaw-dropping Rugby World Cup upset on Saturday.

Relegated to the bench for the Pool A clash, Leitch handed the captaincy to Pieter Labuschagne in Shizuoka but came on after half an hour with Ireland leading 12-3 and gave the Brave Blossoms a huge lift as they came storming back to win.

"It was a complete performance and massive for Japanese rugby," said Leitch, who famously inspired Japan to a famous 34-32 victory over South Africa at the last World Cup.

"Four years ago was very special because we came in as underdogs. But this time the belief was there from the outset we could go out and beat the number-two team in the world."

Ireland, who crushed Scotland 27-3 in their opening game, came in to the tournament as the world's top-ranked side and looked to be in control after tries from Garry Ringrose and Rob Kearney inside the first 20 minutes.

But Japan, driven on by a rampaging Leitch, refused to buckle and fly-half Yu Tamura kept the home side close with three clutch penalties, before a Kenki Fukuoka try just before the hour mark put the hosts in front and triggered wild scenes among a crowd of 47,000.

"Obviously not starting I had a role to perform in the team there so I wanted to come out and make an impact -- I was roaring out there it was so loud," said Leitch, who revealed Japan coach Jamie Joseph had dropped him on the basis of form after their 30-10 win over Russia in last week's Pool A opener.

- 'On the chin' -

"He called me in earlier in the week and said 'Look, you're not starting. The captain needs to be the best player and other players are giving more impact in the game' -- so I took it on the chin," shrugged Leitch.

"That's just another string to our leadership group. It's a shared leadership. My role this week was to prepare the team and it all obviously paid off."

Asked what went through his mind after Fukuoka's try, Leitch smiled: "I went and had a talk to our number 10 (Tamura) because we hadn't finished the game yet -- I was just thinking territory."

Leitch noted that Japan -- who have targeted a first-ever place in the World Cup quarter-finals -- won three games under Eddie Jones in 2015 and failed to reach the knockout stages.

"We still have a job to do," he nodded.

"We talked about it in the locker room. We had 15 minutes in there, had a few beers but that's it -- the party finishes in the locker room," added the 30-year-old.

"Now we go back to the hotel and we look forward to Samoa. But obviously beating Ireland tonight is a massive step -- we've got a game against Scotland and if we do the job there, it will put us in a great position."