Athletic Bilbao's 1-0 victory over Real Madrid caused the latter to slip five points adrift of Spanish league leader Barcelona, which beat Sevilla 4-0 thanks to Lionel Messi's two goals on Saturday.
Athletic's Fernando Llorente's was free in the box to head home his sixth goal of the season after latching on to a corner that was flicked on by Madrid midfielder Lassana Diarra in the third minute.
Athletic goalkeeper Gorka Iraizoz made key saves on Madrid's big three off-season signings — Cristiano Ronaldo, Karim Benzema and Kaka — including a diving save to stop Ronaldo during injury time.
Iraizoz made 12 saves in all as Athletic beat Madrid at San Mames stadium for the first time since 2004.
Madrid seemed poised to equalize as Athletic sat back. Iraizoz stopped long shots from Marcelo and Xabi Alonso before Ezequiel Garay headed a good chance wide not long before the break.
Athletic finished with 10 men when Carlos Gurpegui was sent off for a trip on Ramos in the closing stages, when Ronaldo sent a free kick over goal in Madrid's final chance.
Barcelona's win at the Camp Nou meant the Spanish champions had 46 points to Madrid's 41 after extending their unbeaten start.
"There's a lot of league left, only one half is [nearly] done," Madrid coach Manuel Pellegrini said. "We won't lack spirit or motivation in our title chase."
Following a lacklustre first half, Barcelona got on the board in the 49th when Sevilla defender Julien Escude redirected Gerard Pique's shot into his own goal.
Pedro Rodriguez doubled the advantage in the 70th, chipping over oncoming goalkeeper Andres Palop after being played into the area by Xavi Hernandez.
Messi collected Daniel Alves' cross inside the area before slotting a low shot into goal in the 85th.
The world player of the year completed the rout with his league-leading 14th goal in injury time, the Argentina winger breaking into the area alone before tucking back to beat Palop.
Barcelona, which was knocked out of the Copa del Rey by Sevilla on Wednesday, was stronger in the two teams' third meeting in 11 days as the visitors created little offensively to see their league chances likely end following a fourth straight loss.
Earlier Saturday, two early goals gave Osasuna a 2-0 victory over Espanyol to snap a five-game league winless streak.
At Pamplona, Juanfran Torres beat goalkeeper Cristian Alvarez, who was filling in for Carlos Kameni away on African Cup duty, with a low shot in the fifth minute, before Walter Pandiani headed home a cross three minutes later.
Osasuna were reduced to 10 men with Roberto (Ruper) Lopez's sending off. Facundo Roncaglia wrongly joined Lopez in the stands eight minutes later after the referee showed the Espanyol defender a second yellow, even though teammate David Garcia was the one who fouled Carlos Aranda.
Osasuna improved to 20 points while Espanyol has 19.
In Sunday's games, it's: Malaga versus Getafe; Racing Santander versus Valladolid; Almeria versus Tenerife; Zaragoza versus Xerez; Mallorca versus Deportivo; Atletico Madrid versus Sporting Gijon; and Valencia versus Villarreal.