England forwards coach, and the assistant coach for Japan when they beat South Africa at the 2015 World Cup, Steve Borthwick said the win, which was emblazoned over all the country's newspapers on Sunday, was no surprise to him.
"They play smart, they've got speed, they've got talent," he said.
"Jamie Joseph and Tony Brown are very good coaches. But the big thing is the tenacity with which the team plays. They're very impressive in that regard," he told BBC.com.
Borthwick said the Japanese said were much better physically prepared than previous and were smarter tactically.
"They're good to watch and they play to their strengths.
"We saw when England played them last November how good they are. That first half was a tough half.
"When you see the excitement in the stadium, the passion the supporters have, that's the same wherever you go in Japan," he said.
The former All Blacks, Joseph and Brown, had spent time with coach Eddie Jones and Borthwick when they had Japan at the last World Cup and they spent a week with them in England last year.
Borthwick said, "A big area of development before 2015 was getting the expectation that the team could win, so seeing them go out the way they did on Saturday was great.
"Now they're effectively backing up 2015 and they're building on what has been done there before.
"There's a lot of competition for sport over here. Look at the support baseball gets. Rugby, in a lot of ways, dropped down quite a bit prior to 2015.
"When you look at their World Cup record before then it didn't make pretty reading. The team in 2015 felt the responsibility for the sport in general in Japan, and you saw that growth.
"Rugby's got a long history here. The success of this Japan side could be really fantastic – there will be a lot of young kids watching that game who now want to play rugby," Borthwick said.
England face games against Argentina and France to complete their pool play over the next two weeks.