Barcelona are just three points clear of Atletico Madrid and four ahead of Real Madrid in La Liga as their miserable form at Anoeta Stadium continued with a 1-0 defeat at Real Sociedad on Saturday, a result that blew the title race wide open.
Having been beaten by Madrid in the Clasico last weekend and seen both their arch rivals and fellow title challengers Atletico win earlier in the day, Barca headed to Basque country needing a positive result.
But instead they slipped to a third game without a win as the Catalan club's wait for a first away win over Sociedad since 2007 continued.
Sociedad, who came into the game on the back of a surprise 2-1 win over Sevilla, netted what proved to be the decisive goal in the fifth minute through Mikel Oyarzabal's fine header.
Barca dominated from that point on but, missing the suspended Luis Suarez, could not find a way past the in-form Geronimo Rulli, who pulled off a string of fine saves to frustrate Luis Enrique's men.
Lionel Messi had the best chance to level matters but he was left waiting for his 500th Barca goal as Sociedad held on to secure a repeat of last season's 1-0 win and blow the title race wide open.
Xabi Prieto's excellent delivery from the right was met by Oyarzabal with a superb looping header that left Claudio Bravo with no chance.
Rulli dived brilliantly to his right to turn a fierce Iniesta drive from outside the area behind with a one-handed save.
-Barcelona have not had three winless La Liga games since May 2014 (D1 L2).
-Real Sociedad scored with their only shot on target.
-Leo Messi has not scored for 362 minutes (all competitions), the last time he went through such a long run without scoring was in April & May 2011 (417 minutes).
-This has been the Liga game in which Messi had the most shots without scoring (seven).
-No team leading La Liga with six matches left to play and a three-point advantage above second (in a league with 20 games and three points per win) has lost the title.
