La Liga leaders Real Madrid suffered a surprise 1-0 defeat at mid-table Athletic Bilbao after a bullet header from Aritz Aduriz wrapped up the points.
Aduriz outjumped defender Pepe to power a Mikel Rico cross into the top corner after 26 minutes of a frenetic first-half.
Real have taken just one point from their last two games and, althought they are two points clear, will be overhauled if Barcelona beat visitors Rayo Vallecano on Sunday.
Real were denied time on the ball by Bilbao, who are on a high after booking a place in the King's Cup final with victory over Espanyol on Wednesday.
Bilbao were unable to maintain the pressure after the break and Real began to dominate possession but the visitors struggled to create openings.
Instead Aduriz came within inches of getting a second goal with another header which hit the outside of the post.
Real forward Gareth Bale almost equalised but hit the post with a spectacular effort seven minutes from time when he lobbed keeper Gorka Iraizoz midway inside the Bilbao half.