Barcelona, Spain: Spanish league leader Real Madrid lost 1-0 at Athletic Bilbao on Saturday, stumbling for a second straight week.
Aritz Aduriz rose to head in the only goal in the 26th minute, and strike partner Inaki Williams outshone Madrid's star-studded attack led by Cristiano Ronaldo.
The Ballon d'Or holder was left without a shot on goal, as Madrid's best chance came toward the end when Gareth Bale's long lob hit the post.
Madrid, which drew at home to Villarreal in the previous round, was left two points ahead of Barcelona. The Catalan club can go top by beating Rayo Vallecano on Sunday.
"We gave the game away," Madrid manager Carlo Ancelotti said.
Madrid visits Barcelona's Camp Nou in two weeks.
In contrast with the better-rested visitors, Bilbao's victory came three days after it advanced to the Copa del Rey final against Barcelona. Bilbao rose to eighth place.
Elsewhere, forward Victor "Vitolo" Machin scored twice, earned a penalty, and provoked an own goal to lead fifth-place Sevilla's entertaining 4-3 win at Deportivo La Coruna.
Less than three months ago, Madrid was unbeatable, finishing 2014 with a Spanish record 22-game winning streak. Since then, injuries, tired first-choice players, and tougher competition have brought the European champions down to earth.
Since the winter break, Madrid has lost three times and drawn once in 11 league games, and has also exited the Copa del Rey. It hosts Schalke on Tuesday in the Champions League looking to make good on its 2-0 lead in their round-of-16 tie.
Bilbao got the better of Madrid in a free-flowing match, but in which both teams struggled to create many scoring opportunities.
Ronaldo was held in check, Karim Benzema was barely noticeable, and Bale could threaten only with his desperate long-range attempt as he went without a goal for the eighth consecutive match.
Bilbao, in turn, saw Aduriz net an excellent goal, and Williams, its promising young forward, flourish in his first match against a major opponent with his speed, hustle, and ability to keep the ball under pressure.
"We wanted to make Madrid run backward because you slay giants by wearing them down," Bilbao coach Ernesto Valverde said. "In very little time Williams has become an immense surprise."
Mikel Rico set up Bilbao's goal when he spotted Aduriz's clever run, and lobbed the ball. Aduriz did the rest by out-leaping Pepe to send Rico's delivery between the post and goalkeeper Iker Casillas. Aduriz, who also scored in Bilbao's 2-0 win at Espanyol on Wednesday to send the Basque side into the cup final, has found the net 16 times in all competitions this season.
"It has been a great week, everything has worked out perfectly," Aduriz said.
Madrid appeared to have found the solution with an energetic restart after halftime, increasing the pace of its passes and provoking the first nerves in Bilbao's defense. Francisco "Isco" Alarcon forced goalkeeper Gorka Iraizoz to make his first save in the 58th.
But Bilbao soon settled down, and looked to Williams and Aduriz to tilt the match back in its favor.
After Williams heard "Inaki! Inaki" chanted by the crowd when he was substituted, Aduriz almost scored another goal when he again jumped above Pepe to head the ball off the side of the post in the 76th.
Bale's attempt from near midfield — which almost paid off — underscored the lack of ideas Ancelotti's team had going forward, even with reinforcements Javier Hernandez and Jese Rodriguez.
"We were pretty confused when we were attacking," Ancelotti said. "We were slow, with too many passes. It's not a problem of fitness or attitude. My team played poorly and the responsibility is all mine. The league is going to be tougher, but, with our talent, I believe we can fight for it."
Granada also beat Malaga 1-0 at home to move closer to escaping the relegation zone.
Elche edged Almeria 1-0 at home to emerge from the drop zone and push Almeria down into the bottom three teams threatened to fall come season's end.