Real Madrid suffered a shock 1-0 defeat at lowly Osasuna on Sunday, a setback which leaves them all of seven points behind runaway Liga leaders Barcelona, dpa reported.
The only goal of a tight, intense game was scored by Osasuna midfielder Javier Camunas in the 63rd minute.
Real's second league defeat leaves them with 51 points from 21 games. Barca have a record-breaking 58 points after their 15th straight win on Saturday, 3-0 away to Hercules.
Barca's win in Alicante meant that Real had no choice but to win in Pamplona, where they have not triumphed since 2008. But the whites looked weary after Wednesday's King's Cup exertions against Sevilla, and created few clear chances.
They sorely missed flu victims Xabi Alonso and Marcelo. In addition, Cristiano Ronaldo has suddenly lost form, and Karim Benzema turned in another lethargic display.
The only goal came 27 minutes from time. Osasuna forward Carlos Aranda skilfully brought down a long ball and fed the unmarked Camunas, who scored with a right-foot drive that went in off Iker Casillas' far post.
Real coach Jose Mourinho immediately brought on new signing Emmanuel Adebayor and fit-again Kaka, but the visitors failed to break down a solid Osasuna defence.
In fact, Osasuna came closer to making it 2-0 than Real to equalizing. Six minutes from time the tireless Aranda set up substitute Krisztian Vadocz to score but his shot was cleared off the line by defender Alvaro Arbeloa.
The final whistle was greeted with delight by the Osasuna fans. The shock win takes the Pamplona team up to fifth from bottom.
Earlier on Sunday, the crisis of Atletico Madrid had continued with a 2-0 defeat at home to lively Athletic Bilbao, for whom Gaizka Toquero was the two-goal hero.
The latest defeat leaves Atletico stranded in seventh place, five points behind sixth-placed Bilbao, and with coach Quique Sanchez Flores in jeopardy.
Things started to go wrong for Atletico seven minutes before half-time, when defender Luis Perea was harshly sent off for a foul on Fernando Llorente.
The Spain striker put the resultant penalty wide, but just before the interval Toquero made it 1-0 by turning in a centre from right- back Andoni Iraola.
Atletico, playing with one man less, never looked capable of turning the game around. In the 65th minute Javi Martinez released the tireless Iraola down the right and Toquero converted his centre to make it 2-0.
Sunday's late game was to pitch fifth-placed Espanyol against Villarreal, who are third.