The Samurai Blue took a big stride towards progressing from Group H on Tuesday afternoon by beating Colombia 2-1 in their opening match.
Japan's players celebrated their victory on the Mordovia Arena field after the match, but Nishino acknowledges that there are still tricky tests to come against Poland and Senegal.
"I said at half time that if we can keep the ball moving, we can deprive the Colombia players of their stamina," he told reporters after the match.
"Our players were aggressive from the start and I think that worked very well for us. This is just one win and three points, so we will save our celebration."
Japan had never previously won a World Cup game on European soil in six attempts.