Barcelona lost 1-0 at Real Sociedad on Saturday while Real Madrid and Atletico Madrid both won to gain ground in the title race in La Liga on Saturday.
Sociedad took the lead in just the fifth minute as 18-year-old Mikel Oyarzabal rose to head home Xabi Prieto's cross.
Barcelona failed to find a response without Luis Suarez in the squad and remain without a win at the Estadio Anoeta since May of 2007, a span of seven games.
Lionel Messi had a few chances to score his landmark 500th goal for club and country but was denied by Geronimo Rulli, most notably from close range with five minutes to play.