Real Madrid suffered a huge blow to their Primera Division title bid as they were beaten 1-0 at Athletic Bilbao thanks to a first-half header by Aritz Aduriz.
The European champions lacked their usual intensity and attacking flair, above all in the first half, and drop points for the second match running, following last weekend's 1-1 draw with Villarreal.
They will surrender top spot to archrivals Barcelona if the Catalans beat Rayo Vallecano at home on Sunday.
But while this was another disappointing result for Madrid, it capped a dream week for Athletic, who on Wednesday beat Espanyol to reach the Copa del Rey final.
Madrid coach Carlo Ancelotti made just one change from the team that drew with Villarreal, replacing Luca Silva with Asier Illarramendi.
And just like against Marcelino's side, Madrid struggled to get into the game.
Aduriz had the first shot of the game with barely a minute gone, firing wide from just outside the area and was a thorn in Madrid's side for most of the game, while his young strike partner Inaki Williams also gave a tireless display in attack and defence.
But it was Aduriz who have Athletic a deserved lead after 26 minutes.
Mikel Rico found space in the middle of Madrid's half and floated a diagonal ball into the area and the striker drifted into the area, out-jumping Pepe to head the ball past Casillas and into the near corner.
The 34-year-old is enjoying one of most prolific seasons of his career and this was his 17th goal in all competitions this term.
Madrid's only efforts in a poor first half were a header from Pepe from a corner that missed the target and two poorly struck efforts from Karim Benzema.
Los Blancos were much stronger after the break and spent most of the second half in Athletic's area, but still struggled to create clear chances.
Mikel Balenziaga arrived in the nick of time to boot the ball from Benzema's path after good work by Isco and Cristiano Ronaldo, and then Isco hit the target but was denied by Gorka Iraizoz.
The Athletic goalkeeper, who had a splendid display the last time Athletic beat Madrid back in 2009, was in fine form, also saving from Benzema and Ronaldo, who once again looked a frustrated figure and was unable to add to his tally of 30 league goals for the season.
Athletic had spent most of the half defending but then could have scored a second with 15 minutes left, as Aduriz leaped forward to contest a loose ball and headed off the outside of the post.
Madrid's best chance came out of nowhere, when Bale launched an audacious shot from 50 yards out on the left wing, the ball bouncing off the post across the goalmouth but just beyond the reach of substitute Javier Hernandez.
It was a rare scare for the raucous San Mames crowd, which erupted when the final whistle sounded.