Captain Wataru Endo warned that Japan's Asian Cup rivals will "exploit" their weaknesses after they wobbled before reaching the quarter-finals with a 3-1 win over Bahrain on Wednesday.
Japan looked to be cruising after goals either side of half-time from Ritsu Doan and Takefusa Kubo, but an own goal from error-prone goalkeeper Zion Suzuki let Bahrain back in the game.
Ayase Ueda restored Japan's cushion with his fourth goal of the tournament, but Liverpool midfielder Endo warned that their quarterfinal opponents might not be so forgiving.
"In football, teams are analysing you and trying to find out where there are gaps that they can exploit," he said.
"Teams have looked at us at the tournament so far and it's not easy to defend against.
"It's important that we don't let our concentration drop for the full 90 minutes."
Mitoma came off the bench in the second half to play his first game since injuring his ankle on December 21.
But questions remain for Hajime Moriyasu's side, who were the pre-tournament favourites but have yet to keep a clean sheet in four games in Qatar.
Goalkeeper Suzuki has made a string of mistakes and he was again at fault for Bahrain's goal.
The 21-year-old misjudged a punch before getting tangled up with Ueda and fumbling the ball over the line.
Moriyasu blamed the goal on a lack of communication.
"It wasn't all perfect and next we will play either Iran or Syria. They have a lot of talent up front and we will have to play better than today," he said.
"We have to reflect on the goal we conceded."
Japan opened the scoring in the 31st minute when Seiya Maikuma unleashed a shot from distance that cannoned against the post and Doan reacted fastest to stick home the rebound.
Moriyasu's side were forced into a change when Celtic midfielder Reo Hatate went off injured five minutes later, with Hidemasa Morita coming on.
Kubo doubled Japan's lead in the 49th minute after a VAR goal check.
The Real Sociedad forward looked to be in an offside position when the ball broke loose to him in the box, but the final ball had come off a Bahrain defender.
What looked like being a stroll for Japan threatened to turn sour when Suzuki turned the ball into his own net in the 64th minute.
Ueda calmed Japan's nerves in the 72nd minute when he drilled the ball home after a sweeping move forward.
Mitoma was involved in several late chances after coming on in the 68th minute, but could not get on the scoresheet.
Bahrain bowed out of the tournament after winning Group E ahead of South Korea.
Coach Juan Antonio Pizzi acknowledged that "Japan were better than us tactically and physically".
"Japan have had the same coach for five years and every substitute they brought on had the same level of quality," said Pizzi, who won the 2016 Copa America with Chile.
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