Four months ago Isileli Nakajima had never played prop but on Saturday he packed down in the front-row and helped Japan stun the globe with victory over Ireland.

The Tongan-born front-rower with the gold tooth and the beaming smile climbed off the bench and sustained the Brave Blossoms' scrum ascendancy right until the death.

And when Japan's 19-12 triumph in Shizuoka was complete, Nakajima admitted he hardly knew what to do with himself: the 30-year-old accepting the victory as the pinnacle of his playing days.

Nakajima was battling along as a number eight with Kobe Steelers until Japan's coaches convinced him to make the front-row switch – and the hulking forward insisted he never had any doubts about completing his transition, not even with the complex technical aspects.

"This is my first World Cup and I've only been playing prop for four months, so to play this kind of game at this point and get this kind of win – it's amazing," said a jubilant Nakajima.

"Transitioning to prop was never difficult because we have great coaches, they believe in what I can do. So I just believe them, and stick to their plan.

"We talk every day, for a long time, we have a lot of chats and meetings about how to scrum. So I just believed them, trusted in them, and that's where my confidence has come from.

"It's just incredible. That's the peak of my career.

"We've been working for years towards this specific match, we stuck to our plan, we played well and did a good job. This is Japan's best ever victory, I think."

Japan boss Jamie Joseph revealed he had spent three years devising the gameplan to thwart Ireland, and Nakajima insisted Ireland did just what the Brave Blossoms had planned for.

Japan failed to reach the quarter-finals in 2015 despite three pool stage victories, including that superlative win over South Africa. So now Nakajima and company are taking nothing for granted, with Pool A clashes against Samoa and Scotland still to come.

"Ireland played literally how we expected," said Nakajima.

"We expected they would try to hold the ball as much as they did, so we were just trying to hold them in the defence, then try to get the ball back as much as we could.

"And I think we did it as well as we could. We put a lot of pressure on them whenever we could.

"The atmosphere was really awesome. We couldn't hear each other during the game.

"The people in Japan gave us energy from the first to the last minute.

"In the last few seconds, I knew we were going to win the game, and it was a great feeling.

"We have big confidence but this is not the end for us, we're not going to sit at this and be happy to have beaten Ireland.

"We have a goal so we need to stick to what we've planned for so long for.

"We've got to get to the quarter-final now. It doesn't matter about anything else, we've just got to stick to our plan."