The Germany international is eagerly anticipating a tough semi-final tie against the Spanish giants and is full of respect for Manchester City striker Edin Dzeko
Both sides previously met in the group stages of the competition, with BVB winning 2-1 at home and recording a 2-2 draw in Madrid, and Hummels has stressed Dortmund have learned from their previous encounters about the importance of retaining the ball.
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"I remember our match at the Santiago Bernabeu during the group stage. In the second half we did not have the ball for longer than ten seconds before we lost it again, and then it is really difficult against them."
The centre-back then went on to discuss his most difficult opponent so far and revealed that Manchester City's Edin Dzeko is the attacker who caused him the most trouble.
Hummels took on Dzeko twice in the group stages of the Champions League, with the Bosnia international failing to score on both occasions, but the 24-year-old feels the combination of power and skill makes Dzeko a daunting adversary.
"Dzeko is a very good striker," he explained. "When I played against him it was so difficult because he is so tall and strong but also he has such a good technique and nice control. This makes it very hard to play against him."