Japan manager Hajime Moriyasu was given a surprise shower by a pitch sprinkler on Thursday but it did nothing to dampen his spirits after his team's stunning victory over Germany.

The Blue Samurai came from behind to beat the four-time world champions 2-1 in their Group E opener on Wednesday and are now eyeing a place in the last 16, with Costa Rica their next opponents.

Moriyasu led his team in a workout on Thursday afternoon and was given an unexpected soaking when a sprinkler doused him as he spoke to reporters.

The manager laughed off his early shower but warned his players not to get carried away by their historic triumph.

"All we're thinking about is the next game that's in front of us, the same as we always do," Moriyasu said, stressing that the Germany game was already in the past.

"On the day of a game we celebrate if we win or we feel frustrated if we lose, but we always think of the things we could have done better and start preparing for the next game in a positive way."

Moriyasu said beating Germany had given Japan "peace of mind" for Sunday's game against Costa Rica, who were hammered 7-0 by Spain in their opener.

Costa Rica must win to revive their chances of reaching the knock-out round and Japan defender Yuto Nagatomo said his team will be waiting to exploit any gaps they leave behind.

Nagatomo believes Japan have become a versatile side who are now "difficult for opponents to understand".

"Tactically, we have become a team where it wouldn't look strange if any of us were playing in any position," he said.

"I think we've become hard for our opponents to analyse, almost like a chameleon."

- Big ambitions -

Japan had to dig deep to stay in the game against the Germans, coming back from 1-0 down to win with goals from substitutes Ritsu Doan and Takuma Asano.

The result puts Japan in the driving seat to claim a place in the knock-out round but that will not be enough to satisfy their ambitions.

"It was a big result for Japanese football but we haven't changed history yet," said Doan.

"Our minimum target is to reach the quarter-finals and that's what we've been talking about as a team."

The arrival of Doan and fellow attacking substitutes Asano, Kaoru Mitoma and Takumi Minamino in the second half against Germany helped turn the tide in Japan's favour.

Nagatomo said the injection of pace "lit a fire" under Japan and made them a hit with neutrals inside the Khalifa International Stadium.

"You could see the difference in the mood on the German bench and the Japanese bench," he said.

"The fact that we were all united came across to the crowd and moved them. The stadium started rooting for Japan."

The win was also greeted with raucous excitement across Japan, with crowds taking to the streets and packing into bars to celebrate.

Nagatomo cautioned that a defeat against Costa Rica would pour cold water on the party atmosphere.

"The most important thing is that we don't let off just because we won," he said.

"We have to put that away and move on to the next one. It was a great win but if we lose the next one, it will have all been for nothing."