Espanyol brought Atletico Madrid's 20-game unbeaten run in LaLiga to a juddering halt on Friday as Sergio Garcia's late strike sent Diego Simeone's side to a 1-0 defeat.
Second-placed Atletico had not tasted league disappointment since April when they were beaten by Villarreal, but they offered little on a frustrating night for the visitors at RCDE Stadium.
Atleti only created one chance of note throughout the 90 minutes when Kevin Gameiro fired straight at Pau Lopez, while star striker Antoine Griezmann was well below par.
And their poor form going forward was punished in the 88th minute when a brisk Espanyol counter-attack presented a chance for Garcia, the striker making no mistake with a low finish at the back post.
The win lifts Espanyol five points clear of the bottom three, and will delight the Catalan side's neighbours Barcelona just as much.
Barca's six-point lead at the summit remains intact and could be widened further should they beat Real Madrid in the Clasico, while Valencia could move past Atletico should they beat Villarreal.