Without talisman Lionel Messi, Barcelona slipped to their second defeat in a week as a 1-0 reverse at Athletic Bilbao Sunday blew the Spanish title race wide open.
Already qualified for the Champions League knockout stages, their 2-1 midweek loss to Ajax Amsterdam could be forgiven, but Iker Muniain's 70th minute winner for Athletic has far greater significance.
It was Barca's first league reverse of the season and left them level on points with Atletico Madrid with arch-rivals Real Madrid snapping at their heels a further three points adrift.
Athletic, who are unbeaten at their new San Mames stadium, have moved up into the fourth Champions League spot with the victory.
Muniain might have put them ahead in the first half but he wasted a great chance from close range, while Barcelona offered little in reply.
Brazil's Neymar was kept quiet but just after half time he burst through only to be pulled back by home midfielder Ander Iturraspe, who was lucky to escape with a yellow card.
But the men from the Basque country remained a constant threat and Markel Susaeta crossed for Muniain to slide home his decisive goal.
A second should have followed but Gaizka Toquero headed a cross straight at Barcelona goalkeeper Jose Manuel Pinto, who made a reaction save.
Barcelona coach Gerardo Martino flung on Sergi Roberto and Pedro as late substitutes, but to no avail.
Messi was confined to the sidelines for the second time this season last month with a hamstring injury which is set to keep him out of action until January, missing at least two more La Liga games.
Meanwhile in Italy, champions Juventus went three points clear of early pace setters Roma with a 1-0 win over Udinese Sunday.
It took an injury-time header from Spain striker Fernando Llorente to grab the three vital points as he headed Stephan Lichsteiner's shot past Zeljko Brkic.
Juve had plenty of chances to wrap up their 12th win of the season and sixth in a row, but were frustrated by dogged defense and poor finishing until the last gasp strike.
Earlier, Roma were held to their fourth draw in a row, 1-1 away at Atalanta.
Napoli, who are in third, nine points adrift, travel to Lazio Monday.
Fourth placed Inter Milan failed to make up ground as they were held 1-1 at home to Sampdoria.
Renan scored a late leveler for Sampdoria after Fredy Guarin had given Inter an 18th minute lead at the San Siro.