Keller's side face Los Blancos in the last 16 of the Champions League and will be hoping to take advantage of the Spanish giants' woeful record in Germany, where they have only won once in their last 25 attempts.
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"Behind Bayern Munich, Real Madrid is maybe the second-best team in the world at this moment," Keller told reporters.
"They have extraordinary players in each position and that is why it will not be an easy game for us.
"We'll have to be compact at the back to achieve what might be a minor miracle of getting a result.
"But we are the home team and want to play in a brave way without hiding from Real. To be successful everybody has to play at 100 per cent. Then it might be possible to achieve a little wonder."