Real Madrid goalkeeper Iker Casillas has admitted that Los Blancos cannot afford to make mistakes in the semi-finals of the Champions League.

Madrid entered last night's quarter-final second leg against Borussia Dortmund holding a three-goal advantage from the first leg.

Dortmund won 2-0 on the night following an below-par performance from the Spanish outfit, but Casillas's side still advanced into the final four on aggregate.

"The first goal is an error from us and the second is after losing the ball in the centre of the park," he told AS.

"We had mental strength because with 2-0 in the 37th minute a lot of things go through your head and all this after missing a penalty. The positive thing is that we've learnt to suffer in these games. These errors cannot happen in semi-finals."

The draw for the semi-finals will take place on Friday.