Match ends, Athletic Club 1, Real Madrid 0.
Real Madrid were unable to increase their advantage at the top of La Liga after they were beaten by Athletic Bilbao at San Mames.
Athletic striker Aritz Aduriz, 30, put the hosts ahead with a header from Mikel Rico's cross.
Real dominated the game after half-time and Gareth Bale's audacious long-range effort struck the post.
But Athletic hung on to claim the victory which means Real have just a two-point lead over Barcelona.
The Catalan club will go top if they can take maximum points from Sunday's match against Rayo Vallecano at the Nou Camp.
Real coach Carlo Ancelotti, who guided the club to their 10th European Cup last year, was forced to defend himself against criticism from supporters earlier this week.
Some fans are unhappy with Ancelotti's approach, with media reports claiming they view the Italian as a soft touch.
Real's lead was cut to two points following last weekend's 1-1 draw at home to Villarreal and more dropped points against the Basque club could prove costly for Ancelotti's side.
Athletic opened the scoring when Aduriz escaped the attentions of Real defender Pepe and powerfully headed Rico's cross past keeper Iker Casillas.
After a modest first-half display Real were much improved after the break and Athletic keeper Gorka Iraizoz saved at the feet of Karim Benzema on the hour.
Cristiano Ronaldo appealed for a penalty after he went down under Iraizoz's challenge, while Wales forward Bale's superb long-range effort bounced back out off the post.
Elsewhere, Vitolo scored twice as Sevilla held on to a thrilling 4-3 win away to Deportivo La Coruna.
- 10De Marcos
- 18GurpegiBooked at 78mins
- 16EtxeitaBooked at 39mins
- 17Rico MorenoSubstituted forToqueroat 90+1'minutes
- 7Etxebarria Urkiaga
- 19MuniainSubstituted forSusaetaat 64'minutes
- 30WilliamsSubstituted forFernández Hierroat 75'minutes
- 20AdurizBooked at 30mins
- 22Fernández Hierro
- 26Remiro Gargallo
- 29López Cabrera
- 12MarceloBooked at 86mins
- 24IllarramendiBooked at 41minsSubstituted forRodríguez Ruizat 71'minutes
- 8KroosBooked at 30minsSubstituted forBorgesat 76'minutes
- 23IscoBooked at 74mins
- 9BenzemaSubstituted forHernándezat 80'minutes
- 7Cristiano Ronaldo
- 5da Silva Coentrão
- 20Rodríguez Ruiz
- Alberto Undiano Mallenco
- Shots on Target
Second Half ends, Athletic Club 1, Real Madrid 0.
Attempt missed. Lucas Silva (Real Madrid) right footed shot from outside the box is high and wide to the right.
Substitution, Athletic Club. Gaizka Toquero replaces Mikel Rico because of an injury.
Foul by Jesé (Real Madrid).
Beñat (Athletic Club) wins a free kick in the attacking half.
Marcelo (Real Madrid) is shown the yellow card for a bad foul.
Foul by Marcelo (Real Madrid).
Mikel Rico (Athletic Club) wins a free kick in the defensive half.
Lucas Silva (Real Madrid).
Beñat (Athletic Club) wins a free kick in the defensive half.
Corner, Real Madrid. Conceded by Xabier Etxeita.
Attempt blocked. Jesé (Real Madrid) right footed shot from the centre of the box is blocked.
Gareth Bale (Real Madrid) hits the left post with a left footed shot from more than 40 yards on the left wing. Assisted by Lucas Silva.
Substitution, Real Madrid. Javier Hernández replaces Karim Benzema.
Pepe (Real Madrid) wins a free kick in the defensive half.
Foul by Aritz Aduriz (Athletic Club).
Jesé (Real Madrid) wins a free kick in the defensive half.
Foul by Mikel Rico (Athletic Club).
Gareth Bale (Real Madrid) wins a free kick on the left wing.