Eight members of the Japan squad play their club football in Germany and it was goals from Freiburg's Doan and Bochum's Takuma Asano (28) that gave them the shock come-from-behind victory at Khalifa International Stadium.
"Before I came to the World Cup, people in Germany were saying it would be an easy game against Japan," Doan told reporters on Thursday.
"I gave them a bitter smile while hearing this and I thought 'don't be silly'. As a man, I think the best way to shut them up is with such a result so I'm pleased it turned out that way."
Japan arrived in Qatar determined to improve on their best previous World Cup campaigns by reaching at least the quarter-finals.
Doan said that while the players had enjoyed the victory, they had already turned their minds to their next Group E match against Costa Rica on Sunday.
"Of course, I was very excited. We haven't changed history, but I think it's a historic victory," he added.
"Yesterday my teammates and I were very happy. But starting today, we will put it behind us and prepare for the game against Costa Rica without being complacent."
Veteran full back Yuto Nagatomo (36), who is playing at his fourth World Cup and been to the last 16 twice, put Japan's first victory over Germany down to two factors.
"First, it's because we played with a strong spirit," he said.
"Second, I think we really united as one and fought all together."
The former Inter Milan stalwart also cautioned against complacency even though Costa Rica were beaten 7-0 by 2010 champions Spain in their opening group match.
"They were crushed by Spain in their first game," he said. "But after looking at the footage of their qualifying matches I think everyone in their team works very hard in defence. So the next game will be very difficult."
