Real face their German opponents for the fifth time in the past two seasons in Wednesday's first leg in Madrid, with Dortmund knocking Modric and his team-mates out at the semi-final stage last year.
The Spanish club won the second leg 2-0 at the Bernabeu on that occasion, but the damage had already been done with a 4-1 thrashing at Signal Iduna Park, and the Croatian knows Real have to be more defensively solid if they are to progress this time around.
"We'll do our best to beat them," he said. "It will be a tough match and it's important for the return leg that we don't concede.
"After the four games we played against them last year we know how dangerous they can be on the break.
"Until the two defeats (against Barcelona and Sevilla last month) we were having a fantastic season. We're the same players and the same team now.
"We are really motivated to put last season behind us and we have to focus on this match. We can't lose ours because we lost against them last time.
"We lost the first leg last year. We didn't expect there to be such a difference and we nearly went through in the second leg.
"I hope to be in the semi-finals. We have to be focused in attack and defence."
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