Rory Best believes Japan's stunning Rugby World Cup victory over Ireland should not be seen as a massive shock due to the talent of the host nation.

The Brave Blossoms, who upset South Africa in their 2015 World Cup opener, lit up their home tournament on Saturday by beating the world's second-ranked side 19-12 in a classic in Shizuoka.

Ireland captain Best, while acknowledging his side's mistakes, paid tribute to Japan and insisted there had been no complacency on the part of the favourites.

"We knew how tough it was going to be," he said. "Anybody who was utterly shocked hasn't seen how good they are.

"We came in with a game plan and we felt we were really prepared, but give credit to them, they played really well.

"We made a few mistakes and were on the wrong side of the penalty count, but Japan played really well, posed a lot of questions to us and unfortunately we couldn't come up with the right answers."

Coach Joe Schmidt said his side "stopped playing" after moving in front through tries from Garry Ringrose and Rob Kearney, although he was frustrated by the officiating.

"We played well in the first quarter," said the Ireland boss. "We got out to a 12-3 lead and [then] we stopped playing to a degree.

"It was pretty difficult and we probably fell on the wrong side of the penalty count - three or four of them were offsides that we felt were pretty tough. That could have gone either way - particularly with some of theirs.

"It gave them the front foot and they didn't button off. Congratulations to Japan, what a furious, intense effort it was. I guess we knew that was potentially coming

"We're incredibly disappointed we didn't manage to control the end of the game. They're a tremendous side, they did really well, and congratulations to them."