Japan coach Hajime Moriyasu watched his side achieve a stunning 4-1 win over Germany on Saturday and then said they "still need to improve" before the Asian Cup next year.
Japan scored two goals in each half to thump their struggling hosts, pushing Germany manager Hansi Flick closer to a possible exit just months before his nation hosts Euro 2024.
It echoed Japan's 2-1 win against the Germans in last year's World Cup in Qatar, which helped dump Flick's team out of the tournament at the group stage.
Moriyasu told reporters "we need to not only congratulate ourselves, but we need to continue our development."
"There are more teams, more challenges, so in this sense, we have many more tasks."
The coach said his side "put a good deal of pressure on Germany" but "we need to improve."
"We need to be more compact, everyone must defend, everyone needs to attack, we need to react dynamically."
The four-time Asian Cup winners have now won three games in a row ahead of the tournament, which kicks off in January in Qatar.
Moriyasu said his opponents were "still among the top teams in the world" despite the loss, and credited Germany and the Bundesliga for improving Japanese football in recent years.
"There are always good times and bad times... we are in the middle of a good period.
"I want to make it clear, we have had lots of support from Germany and German managers... many players have learned a lot during their time in the Bundesliga.
"We are very grateful to Germany."
Japan won praise from Flick, who said his side could learn from the Blue Samurai's recent development and improvement.
"You can see how much they've developed and how much football has developed in Japan.
"You can see how skilled they are, offensive and defensive. We need more."
Japan take on Turkey on Tuesday in their final friendly of the international break.