Barcelona lost its second straight game without injured star Lionel Messi when Athletic Bilbao handed the defending Spanish league champions a 1-0 loss Sunday at San Mames.
Forward Iker Muniain scored the winner in the 70th minute after Bilbao's incessant pressure recovered the ball and sparked a counterattack inside Barcelona's half.
Neymar was once again tagged to pick up Messi's goal-scoring duties, but when he wasn't frustrated by slippery footing Bilbao's defence got the better of him.
Barcelona was left level on points with Atletico Madrid and three points ahead of Real Madrid after both title rivals won on Saturday.
Barcelona's first defeat in the domestic league following 13 wins and one draw came five days after it lost 2-1 at 10-man Ajax in the Champions League.
Barcelona hadn't suffered consecutive defeats since March.
"This game had nothing to do with the one at Ajax," said Barcelona midfielder Sergio Busquets. "Two losses have come in a row, but we have to keep going forward. Sure we made mistakes, but we also did things well. We played a good, wide-open game against a great rival that knew how to pressure us."
Gerardo Martino's team has won once and lost twice since Messi suffered a left hamstring injury that will likely keep him out until January.
Valencia, Sevilla earn wins
Also on Sunday, Valencia beat 10-man Osasuna 3-0, Sevilla won 2-1 at Granada, and Rayo Vallecano drew 2-2 at Real Betis.
The match was billed as perhaps the toughest test of the season for Barcelona coming off its first setback in 21 games and without the injured Messi, goalkeeper Victor Valdes, and defenders Dani Alves, Carles Puyol and Jordi Alba.
Bilbao, meanwhile, has yet to lose in seven games at its new San Mames Stadium. The victory lifted it into fourth place and in the final Champions League spot.
Like so many times in past meetings between the Basque and Catalan clubs, the pace remained frenetic and intense with the ball constantly changing sides.
Early on, Barcelona coolly worked its way through Bilbao's pressure and just past the 10-minute mark deployed a number of one-touch passes in a series of attacks that went unrewarded.
Neymar first had his strike palmed over the bar by Bilbao goalkeeper Gorka Iraizoz in the 12th before Andres Iniesta spoiled a back-heeled flip by Busquets by firing wide.
Then it was Bilbao's turn and Muniain had a golden chance to put the hosts ahead in the 29th, but he barely made contact on a deflected cross that reached him with only goalie Jose Pinto to beat.
Barcelona looked to hit Bilbao on the break, but Neymar slipped several times while trying to dribble and had to change his shoes in the 34th.
The new boots still didn't help the Brazil striker in the 50th when he wasted Cesc Fabregas' pass by sailing his shot high, and his frustration at Carlos Gurpegui's marking boiled over and soon earned Neymar a yellow card when he retaliated.
Neymar then drew a yellow card for Andre Iturraspe when he fouled the Brazilian from behind. Neymar curled the resulting free kick by the near post in the 59th.
Bilbao kept up its pressure and it finally paid off when Ander Herrera stole the ball and passed to Markel Susaeta who crossed for the streaking Muniain to stab in the goal before Barcelona could recover in defence.
Barcelona never threatened to equalize, and Bilbao striker Gaizka Toquero almost added a second goal when he headed into the body of Pinto in the 74th.
"We said all week that we would play with the same intensity as always," Toquero said. "We also had the good fortune they missed their chances. We pressured them well, stole a good number of balls and didn't let them play their game."