The Catalan outfit will enter the clash at Bernabeu in second position in the La Liga table, eight points behind the leaders Real Madrid with seven games left.
A win for Barcelona would cut the gap to five points with six to play, but a defeat would leave them 11 points off the summit and all but end their hopes of securing the trophy for the second successive season.
Xavi is expecting a difficult challenge against Real Madrid, with Carlo Ancelotti's side knocking Manchester City out in the quarter-finals of the Champions League on Wednesday night.
Barcelona, though, were unable to progress to the final four of the European Cup, with Paris Saint-Germain recording a 4-1 victory in the second leg of their quarter-final to advance.
Xavi expecting tough challenge in El Clasico«Tomorrow we have the chance to get back into La Liga. It happens to be the most important game of the season. We will find a strong Madrid and even more so after eliminating the best team in the world, City,» Xavi told reporters during his pre-match press conference.
«The euphoria will be sky high and the opposition will be very strong. Today we trained well, yesterday we had a Champions League hangover. The team has all the enthusiasm in the world to fight for this league.
»Madrid had to defend against City, who are the best team in the world today. They had to defend and they defended very well. These are situations that you have to dominate and they also know how to do it.
«In the first half with us, they did well. They have potential, they know how to attack and make transitions.
»We talked about it today, that in the first few minutes of each half,
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