Real Madrid suffered a shock defeat by Alaves on Saturday to make it four games without a victory and also four games without a goal.
For the European champions, it was their longest scoring drought since 1985, as Manu Garcia headed home in the 95th minute to send the Mendizorrotza Stadium wild.
It is the first time they have beaten Madrid here since 1931.
Alaves fans ran onto the pitch to celebrate their stunning 1-0 win and the chants continued long after the final whistle.
For Madrid, this was another body blow and to make matters worse, Gareth Bale and Karim Benzema, their chief attacking threats, were both forced off injured. Bale had missed the Champions League loss to CSKA Moscow on Tuesday with a groin strain.
It means Barcelona can go three points clear if they win their game in hand against Valencia on Sunday while Alaves’ excellent start to the season continues. They move up to third, level on points with Real.
However, Madrid coach Julen Lopetegui insisted it is too early to be questioning his future despite the strings of failure.
Lopetegui had been asked on Friday if he felt under pressure at the club he left Spain for on the eve of the World Cup and the question was put to him again after the match here.
“It interests you but is not a question that concerns me,” Lopetegui said.
“We know what the life of a coach is, especially at Real Madrid, but we also know that it is October and you have to keep perspective and stay calm.”
Talk of change is perhaps premature but it will only increase if Madrid do not find fluency soon and, in particular, if Barca pull away. The Clasico at the Camp Nou looms at the end of the month.
“We are disappointed,” Lopetegui added. “In the end, defeat is a harsh punishment.”
Thibaut Courtois took over from Keylor Navas in goal and was at fault for Alaves’ winner. The Belgian failed to claim the cross, leaving Garcia to head into any empty net.
Bale had been passed fit to start while Luka Modric and Sergio Ramos returned after being rested against CSKA. The first half was a fight, full of heavy touches, tussles and collisions but largely vacant of clear-cut chances.
Ramos was at the heart of things as he clattered Jonathan Calleri and then elbowed the Alaves striker in the face. The referee deemed it accidental but Calleri clearly did not, responding soon after with a clip on the defender’s leg.
Bale span in behind and backheeled to Benzema, who teed up Dani Ceballos to shoot. Benzema had a header volleyed off the line by Victor Laguardia.
But Alaves grew into the half and it took Raphael Varane’s diving header to prevent Ibai Gomez from tapping in at the back post.
Benzema was replaced at half-time by Mariano Diaz, who instantly looked more dangerous.
Only a dangling Laguardia leg ended his weaving run into the penalty area while Real right-back Alvaro Odriozola broke through but failed to find a team-mate with the pull-back. Lopetegui threw his arms up in frustration.
The rain came down and the whistles grew louder as Madrid dominated the ball but Alaves could have won it sooner when Jony sprinted clear, only to drag his finish wide.
Bale trudged off off and another Alaves chance arrived in injury time. Courtois came for the cross but failed to gather and Ruben Sobrino sent the ball to the back post. Garcia was perfectly placed.