Captain Maya Yoshida warned Japan they will have no chance at the World Cup if they "fall apart after conceding one goal", following a sobering 3-0 home defeat to Tunisia.
The four-time Asian champions ended an encouraging run of results on a sour note on Tuesday with a second-half collapse in Osaka.
The experienced Yoshida gifted Tunisia a penalty in the 55th minute and made another defensive howler just over 20 minutes later as the visitors – who are also going to the World Cup – doubled their lead.
A third Tunisian goal in injury time completed a miserable evening for Japan and Yoshida warned a repeat is likely to be fatal in Qatar, with Germany, Spain and Costa Rica their opponents in an ominous Group E this winter.
"We have to learn our lessons from this game. The things we need to address are clear and we need to get to the bottom of them," said the 33-year-old Sampdoria defender.
"We need to be sure that we know where we can and can't make mistakes."
Yoshida said Japan's game "fell apart" after he scythed down Taha Yassine Khenissi in the box and Mohamed Ali Ben Romdhane slammed home the penalty.
The Japan captain was again at fault for Tunisia's second, allowing Youssef Msakni to sneak in and dispossess him before passing to Ferjani Sassi to rifle home.
Substitute Issam Jebali added a late third with Japan stretched, but Yoshida said the game had been lost long before then.
"It's tiring when you concede a second goal against an opponent like this," said Yoshida.
"We can't fall apart after conceding one goal. We can't forget to dig in when we're 1-0 down."
Japan had been buoyant going into the game after beating Paraguay and Ghana 4-1 and losing only 1-0 to Brazil in friendlies, just five months before the World Cup.
Manager Hajime Moriyasu said he was pleased he had increased his "options" after trying out several players and tactics during the four games.
"It's not just the games, the whole time we have been together has been important," he said.
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