Three days after exiting the Europa League, Barcelona had a rare setback in La Liga after losing 1-0 at Almeria.
Barcelona had the chance to extend their lead to 10 points over Real Madrid, but the Catalan giants instead flopped to their first-ever loss to Almeria in their 16th meeting.
The leaders were unable to mount a credible fightback despite having ample time to respond to striker El Bilal Touré's goal in the 24th minute.
Xavi's men will now have to regroup ahead of Thursday's visit to Real Madrid in the semifinals of the Copa del Rey.
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