Barcelona crashed to their first La Liga defeat of the season and failed to restore their three-point lead over Atletico Madrid at the top when they were beaten 1-0 at Athletic Bilbao.
Barca were again without the injured Lionel Messi on Sunday, who is not expected back until mid-January, and turned in one of their worst performances of the campaign against a fired-up Bilbao at the Basque club's new San Mames stadium.
A miserable week for the Spanish champions, which included Tuesday's 2-1 Champions League loss at Ajax Amsterdam, went from bad to worse when Bilbao forward Iker Muniain stole in to net from close range in the 71st minute.
Barca barely mustered a shot on target against the Basques, with Brazil forward Neymar particularly disappointing, and it was only the second time in 15 matches this season they have dropped points after the 0-0 draw at Osasuna in October.
Gerardo Martino's side are level with Atletico on 40 points following the Madrid club's 2-0 win at promoted Elche on Saturday, while Real Madrid, whose 4-0 drubbing of Real Valladolid on Saturday included a hat-trick from record signing Gareth Bale, are three points further back in third.
Bilbao ended a streak of 14 winless La Liga games against Barca and beat them for the first time since May 2006 to climb above Villarreal into fourth on 29 points.
"It was an open game against a great opponent who pressed us hard and knows how to play," Barca midfielder Sergio Busquets said in a pitchside interview with Spanish television broadcaster Canal Plus.
"They had their moments and we had ours but if we don't take our chances then it becomes harder," added the Spain international.
"All we can do is congratulate Athletic and carry on because there is no other choice.
"We have to learn from the mistakes we made, which for sure were numerous, but also from the positives."