Japan weren't expected to reach the Rugby World Cup quarter-finals, but they have confounded their critics and surprised the sport by unleashing a "unique" style -- a combination of high-speed attack and defensive tenacity.

After Japan won four out of four to top Pool A, the rugby world has been awash with praise for the one-time whipping boys, who have been forged into a formidable unit by their coach Jamie Joseph.

The ex-All Black brought together a squad that features an almost equal split of native Japanese and foreign-born players who have qualified for their adopted country through residency.

It is a mix of hardened veterans alongside some serious speedsters, who have been able to produce tries out of nothing but also dig in and defend doggedly when they have to.

Few would have predicted that Japan would go unbeaten in a pool that also contained Six Nations giants Ireland and Scotland, and a potential banana skin in Samoa.

They underlined their qualities in Sunday's 28-21 win against Scotland, when a blitz of four cracking tries was followed by a defensive rearguard as they repelled a Scottish fightback.

"In that first half, there would have been few sides able to stop Japan scoring a couple of tries," Matt Dawson, a World Cup winner with England in 2003, wrote in his BBC column.

"They play with such intensity and their ability to handle the ball at such pace takes an incredible amount of accuracy."

Ex-Australia playmaker Matt Giteau, currently playing for Suntory Sungoliath in the Japanese Top League, added: "This is magic from Japan!

"This is so so huge for the country & a life changing moment for the players .. What an amazing night for rugby in general!!"

"Dark places"

The Japanese pace, allied with the ability of half-backs Yu Tamura and Yutaka Nagare to exploit the shortside before changing the direction of play, was crucial to the hosts' attacking gameplan.

Such tactics, however, mean teams tire. Japan did just that, letting Scotland back into the game with two second-half tries.

But the team skippered by the inspirational flanker Michael Leitch dug deep and showed another side of their make-up: a mental fortitude and defensive tenacity that effectively shut down the game.

It is not easy to train such combative cohesiveness, but Japan's veteran lock Luke Thompson explained that everything had been well planned.

"We have been to some dark places in preparation -- pretty tough camps and that is modern rugby," said the 38-year-old Kiwi-born second-rower, who is playing his fourth World Cup.

"You don't go out and do that stuff by accident, you have to earn it."

Joseph said there was no doubt his players had benefited from exposure to Super Rugby with Japan's Sunwolves outfit.

"It was the tenacity, I guess, of our team at crucial parts of the Test which helped," he said.

"Persistence and confidence and trust the plan is what saw us through. In the last two or three minutes it was a Test match we didn't want to lose."

Dawson said Japan were playing a "unique" style.

"Not even the All Blacks play at some of the speed they do," the former scrum-half said.

Scotland coach Gregor Townsend predicted a "very tough game" for South Africa in the quarter-finals.

"Japan play a really fast game and create quick ruck ball at the ruck and they execute their set piece really well," he said. "It will be a very close game."

© Agence France-Presse