City's elimination at the Champions League round of 16 level last night means Real Madrid have now won five of the past six ties the two clubs have met.
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But when asked afterwards if Real are his toughest rival, Guardiola replied: "My biggest challenge has been Jürgen Klopp at Liverpool. You were in Spain and you have not realised what that competition against them was."
He then added sarcastically in press room about losing at home against Real Madrid: "They have eliminated more qualifying rounds than we have done.
"But it's how we played... I think they know it too. Congratulations."