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.
The four-time champions will play either Iran or Syria after a last-16 victory in Doha that saw Brighton winger Kaoru Mitoma return from injury. 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 quarter-final 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. We have to reflect on the goal we conceded," he said.