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.
Forward Iker Muniain scored the winner in the 70th minute on a counterattack, taking a cross from Markel Susaeta before Barcelona could recover in defense.
That left Barcelona even with Atlético Madrid and three points ahead of Real Madrid after both teams won on Saturday.
It was Barcelona's first loss in La Liga, following 13 wins and one draw, but it came five days after the team lost 2-1 against Ajax in a Champions League match.
Barcelona hadn't lost two games in a row since March.
"When you lose there are always doubts and questions that arise, but one game had nothing to do with the other," Barcelona midfielder Andrés Iniesta said. "We have to correct our mistakes. It's a loss. That can happen. We shouldn't see it any other way."
Messi has been ruled out until January with a leg injury.
In other European club action, Chelsea took over second place in the Premier League on Sunday, beating Southampton 3-1 after giving up a goal to Jay Rodríguez 13 seconds into the game.
John Terry scored in his 400th Premier League appearance after Gary Cahill offset Southampton's opening goal by Rodríguez at Stamford Bridge. Substitute Demba Ba added a goal in the 90th minute.
Chelsea trails leader Arsenal by four points.
In Turin, Italy, Fernando Llorente scored during stoppage time to help Juventus defeat Udinese 1-0 and stretch its Serie A lead to three points.
While host Paris Saint-Germain restored its four-point lead over Lille at the top of the French league by beating Lyon 4-0.
Edinson Cavani opened the scoring by heading home a corner from Lucas Moura in the 36th minute before Zlatan Ibrahimovic converted a penalty in the 41st.
Thiago Silva added a third goal from close range in the 60th and Ibrahimovic sealed the win from the penalty spot in the 83rd.
Ibrahimovic is now the top scorer in the league with 11 goals, one more than Cavani, as PSG stretched its unbeaten run to 36 games in all competitions.
Lyon dropped to the middle of the table, 18 points behind PSG.
Follow us on twitter.com/foxnewslatino
Like us at facebook.com/foxnewslatino