Reigning champions Valencia lost further ground in the title race with a 1-0 defeat away to Athletic Bilbao in an untidy league clash on Sunday.
With the top three all winning on Saturday, Valencia were badly in need of a similar result to stay in the hunt for a successful title defence.
Joseba Etxeberria's brilliant run and chip settled the game in Bilbao's favour, though, and left Valencia in fourth place, now eight points behind leaders Real Madrid.
Rafa Benitez's side, who host Arsenal in an all-important Champions League clash on Wednesday, are also being pressed from behind after fifth-placed Celta Vigo won 1-0 away to Atletico Madrid thanks to a late strike from defender Eduardo Berizzo.
Barcelona failed to make any progress in their quest for a European place, though, the Catalans turning in one of their worst performances of a wretched domestic season in a goalless draw at struggling Alaves.
Real Madrid lead the standings with 54 points from 26 games following their 1-0 win away to Villarreal on Saturday.
Real Sociedad stay second on 53 points thanks to their 2-1 victory away to Racing Santander, while Deportivo Coruna keep hold of third place two points further back after their 2-0 success at home to Rayo Vallecano.
Valencia are fourth on 46, now just five points ahead of Celta Vigo and with 12 games still to play.
Barca remain down in ninth place on 33 points -- 13 points behind Valencia in the fourth and final Champions League spot, 21 points off the title pace and just four points above the relegation zone.
Victories for the top three teams on Saturday left Sunday's focus on fourth place and Celta put the pressure on Valencia with their valuable 1-0 win at Atletico.
A match dominated by the two defences was settled with a late counter-attack started by Gustavo Lopez. He switched the ball across to Berizzo and the Argentine produced a precise left-foot finish to give his side the points.
Valencia failed to respond to the challenge despite being awarded a penalty on 38 minutes when Bilbao defender Aitor Karanka held down Ruben Baraja at a corner.
Fabio Aurelio saw his spot kick saved and within a minute the home side were ahead, as Santi Ezquerro fed the ball to Ismael Urzaiz, who in turn picked out Etxeberria to lift a shot over Santiago Canizares.
Valencia had a couple of chances to get back in the game but their task was complicated when Mista was sent off for a second yellow card 10 minutes into the second half and Bilbao held out comfortably enough.
Barcelona's draw at Alaves was at least enough to preserve the unbeaten record of replacement coach Radomir Antic but there was precious little else to cheer for the Catalans.
Patrick Kluivert wasted their one clear chance, striking the bar with his chip over the goalkeeper in the first half.
Barca had Thiago Motta sent off just past the hour and they concentrated on frantic defence thereafter.
Alaves had Oscar Tellez red-carded two minutes into injury time for handball and Juan Roman Riquelme blasted the free kick over the bar to bring a sad end to a dreadful game.
Reuters