Eight of Japan's World Cup squad play in the Bundesliga but forward Ritsu Doan said on Saturday that friendships are on hold when they meet Germany in their opening game in Qatar.
Japan start their group E campaign against the Germans on Wednesday and both teams are likely to come up against some familiar faces from their club sides.
Doan could square off against Freiburg teammates Matthias Ginter and Christian Gunter, while Japan's Eintracht Frankfurt forward Daichi Kamada could be reunited with Mario Gotze and Kevin Trapp.
Doan, who joined Freiburg from PSV Eindhoven in the summer, said relationships will go out the window when they step onto the pitch in Doha.
"I don't contact them when I'm here. I spoke with them before I left Germany," said the 24-year-old.
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"Of course, I'm happy to play against them but we're not friends anymore until the new year. I will give everything for my country."
Doan believes Japan have "a big chance" of progressing from a tough group E that also includes Spain and Costa Rica.
He thinks inside knowledge can give them the edge over Germany, and teammate Yuto Nagatomo said the Bundesliga-based players have not been slow to share their information.
"You don't even have to ask – in the canteen or on the bus, you're getting information from everywhere," said Nagatomo.
"I've seen a lot of videos, so many that I don't even need to watch anymore because it's all there in my head."
Kamada has emerged as a key player for Japan in recent months and has burnished his reputation with Frankfurt, scoring seven league goals this season.
Doan described his teammate as "an important player" and said Japan have no reason to feel inferior against the Germans.
"Germany have many players from Bayern, and we play against Bayern, so I know the individual quality from them," said Doan.
"Of course everyone knows they are good but I think we should play with confidence and believe in ourselves."