The Basque side lost for just the second time in the league this season
Yangel Herrera scored the only goal of the game in the 77th minute as Espanyol beat Real Sociedad 1-0 on Sunday to dent the Basque side's title aspirations.
La Real would have returned to the top of LaLiga with three points but instead were beaten for just the second time this season in the Spanish top flight.
They were unbeaten in 13 games going into the match, dating back to their opening day defeat at Camp Nou against Barcelona.
Espanyol, meanwhile, continued their excellent home form as they moved up to ninth in the table. They have 17 points at home this season, the same as Barça, while only Rayo Vallecano have more (19 points).
Real Sociedad will hope to get back on track next week when they host Real Madrid at Anoeta, while Espanyol travel to high-flying Rayo.