Kobe - Former Japan assistant coach Steve Borthwick praised the tournament hosts' "tenacity" following their shock Rugby World Cup win over Ireland, predicting it would boost the growth of the game in the country.

Japan's 19-12 victory over Ireland in Shizuoka on Saturday followed their "Miracle of Brighton" defeat of South Africa at the 2015 World Cup in England and leaves them eyeing a maiden quarter-final place.

Back then, Eddie Jones was Japan's head coach and Borthwick his deputy, with the duo now holding similar positions with England at this year's World Cup.

But Borthwick still keeps an eye on the Japanese team and watched their stunning win against the Irish on television.

"They played very well," Borthwick told reporters at England's hotel on Sunday.

"I think it's really important for Japan they are effectively backing up 2015, building on what has been done before."

The former England captain added: "I think it is good for the competition as well, good for the World Cup to see the home team do so well."

Jones and Borthwick were widely credited with instilling a winning mentality in a Japan team that previously lacked the self-belief to challenge the sport's major powers.

"That was certainly a big area of development prior to 2015, the team believing they could win and having the expectation of a win," explained Borthwick, who said the 2015 exploits "undeniably" lifted rugby in the country.

Borthwick credited current coaches Jamie Joseph and Tony Brown, two former All Blacks who both played club rugby union in Japan, with driving the team further forward.

"The team played with tenacity and it was very impressive. They have speed and talent, we know that."

Saturday's win helped Japan climb to an all-time high of eighth in the world rankings and Borthwick said their rise was "great for World Rugby" and "an important reward for the work being put in."

For all Japan's traditional fast-paced handling game has won plaudits, there has long been a perception they can be overpowered physically by more established nations.

But having witnessed how competitive Japan were in a 35-15 loss to England at Twickenham in November, where they led 15-10 at half-time, Borthwick said they now posed a threat in all areas.

"Physically they are very good. I think the rugby is good to watch. They play to their strengths," he said.

The coach said one of the things that had changed in Japanese rugby was their mindset. "There is an expectation now that Japan can win."

Their results before 2015 do not make "pretty reading," he said -- Japan were on the end of a 145-17 hiding from New Zealand in 1995, still the most points conceded in a World Cup match.

He predicted the Shizuoka shock would drive the development of the sport further, noting how stadiums have been packed out around the country for World Cup matches.

"When you see the excitement in the stadiums with their supporters they have, the passion for the game is there," he added.

"As more and more people are exposed to the sport during this tournament, I think it can grow even bigger."