Dortmund beat Real Madrid 2-1 on Wednesday night and have a four-point lead over third-placed Ajax, and Hummels is keen to stay focused on the remaining three games.
"Today was a completely different game to the derby against Schalke this past Saturday," UEFA.com quotes Hummels as saying. "It was a fantastic game, with a fantastic atmosphere and a perfect result.
"We didn't take the criticism we received last year too seriously but also we won't allow this success to go to our heads either. We still haven't won anything yet."
The German champions are the only team in Group D without defeat so far.