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.
Espanyol did a good job of frustrating their in-form visitors in the early exchanges and had the clearest opening of the half after 19 minutes as Baptistao was played clean through.
Moreno won the ball back on the halfway line and released Garcia to drive at the Atleti defence.
His pass found Baptistao in acres of space but, when one-on-one, he got his attempted lob horribly wrong and the ball sailed wide of Jan Oblak's goal.
It was a rare threat on the Atletico goal and slowly the visitors began to up the tempo. However, testing Pau Lopez proved difficult.
Indeed, Atleti's only dangerous moment came five minutes before the break as Fernando Torres' block fell for Griezmann in the area.
The striker tried to go round Pau Lopez but instead of shooting fell theatrically to the floor and was lucky to avoid a booking from referee Mario Lopez.
It took until the 53rd minute for the first opening of the second half to arrive and again it was Espanyol who threatened, Moreno and Garcia involved as the latter screwed a low shot from the edge of the area wide.
The ball finally nestled in the net moments later as Diego Godin powered a header beyond Pau Lopez, but the Uruguayan's effort was ruled out for a foul.
A lacklustre Atletico were eventually able to call Pau Lopez into action when Griezmann played in Gameiro, however the substitute's effort was easily saved to sum up a disappointing night.
And Garcia made it even worse for the title contenders with his first goal of the season, curling a low shot past Oblak to give Quique Sanchez Flores victory over his former club.
