Bayern are currently on a midseason camp in Qatar as they prepare for the second half of the season, and Muller says a match in training is tougher than that of their league opponents.
"We have a very high level in training," Muller told the DPA news agency.
"The defending team are part of Bayern and it is often more difficult to win against our training team than perhaps against a team in the Bundesliga.
"[This is] precisely because the tactical level is identical because, of course, everyone has the same understanding," added the German international.
Bayern currently lead the Bundesliga standings by 11 points, however, they face a tough challenge ahead as they look to advance in the Champions League knockout phase as well as the German League Cup.
Muller believes their Champions League Last 16 tie against Shakhtar Donetsk will bring a tougher challenge as well as their DFB-Pokal test against Eintracht Braunschweig.
“It looks good in all competitions, but in March and April it will get very interesting when it comes to the crunch," he added.
"A Champions League title you do not get cheaply."