The world champions played six friendlies between last November and the start of the tournament in Russia, winning only one, against Saudi Arabia.
Mueller, the Bayern Munich striker who is appearing at his third World Cup, admitted Germany will need to up the tempo in Saturday's second Group F game against Sweden in Sochi.
"I think we took things a bit too lightly after the warm-up games," Mueller told reporters. "Internally we made no bones about it, we were self critical and said we need to do this, that and the other.
"And in warm-up games we had just hoped that in a non-training camp context, without tough and lengthy training sessions, we hoped that when the World Cup came around we could raise our game. And we haven't been able to. We need to get back to being prepared to walk that extra mile. That's it.
"What is important now is that we look ahead to the two major tasks lying ahead of us in this group. The heat is on."
Germany lost their opener 1-0 to Mexico on Sunday in what was one of the biggest shocks of the World Cup so far.
The Germans, so brilliant in winning their fourth World Cup in Brazil four years ago, looked shocked at the Mexican blitzkrieg and although they came into the game in the last quarter, it was too little too late.
Mueller and German team manager Oliver Bierhoff stressed they knew improvement was needed but said they would stick to the principles that have brought them so much success in the past.
"In analysis it's all too easy to blame somebody or just one aspect," Bierhoff said. "I don't think it is a matter of attitude that players don't want to perform, that is nonsense.
"It is just that in the first half (against Mexico) it was very, very visible that players weren't willing to go the extra mile, there was always one yard missing. But I think they have realised that 99 percent is not enough."
"All of the players in the squad will show a reaction," he added.
(Reporting by Andrew Downie; Editing by Christian Radnedge)
