On Saturday, champions Barcelona were 3-1 victors at Real Madrid and their city neighbours stormed into an early lead over visiting Atletico.
Joan Verdu scored twice inside the first eight minutes and Ndri Romaric added a third in the 20th before Atletico were able to react.
Former Porto striker Radamel Falcao bravely headed one back for Atletico in the 32nd minute, but Sergio Garcia's determination saw him bundle in a fourth for Espanyol after the break.
Arda Turan pulled one back for Atletico with a late volley.
Espanyol climbed to eighth with 20 points from 15 games, one ahead of Atletico in 10th, but 17 adrift of their derby rivals Barca in first place.
Pep Guardiola's side moved level with Real on 37 points but top on goal difference, having played a game more, after coming from behind to beat their arch-rivals at the Bernabeu.
Big-spending Malaga are sixth on 24 points after being held 1-1 at home against Osasuna despite creating a raft of chances.
With almost their first sight of goal the visitors scored when Ibrahima volleyed a loose ball into the roof of the net in the 33rd minute. Juanmi levelled after a slick passing move in the 70th.
Seventh-placed Osasuna with 22 points are three ahead of Athletic Bilbao who conceded a stoppage-time header against Racing Santander to draw 1-1 at home.
Marco Ruben rescued a point for misfiring Villarreal as they could only draw 1-1 at home with 10-man Real Sociedad.
Villarreal, who dropped out of the Champions League during the week, fell behind just after the break at the Madrigal.
Mikel Aranburu's solo effort was soon followed by a red card for midfielder Gorka Elustondo.
Argentine striker Ruben slid in at the back post 10 minutes later to level for Villarreal, who are missing the goals of the injured Giuseppe Rossi, but they could not find a winner.
Villarreal lie 17th in the standings with 15 points and are only kept out of the bottom three by their superior goal difference over Sporting Gijon.
Rangers striker Nacho Novo scored twice to earn Sporting a 3-1 win at promoted Rayo Vallecano, who slumped to their fourth defeat in a row.
Elsewhere, Getafe overcame promoted Granada 1-0 at home with a late strike from Francisco Casquero.
Real Zaragoza's miserable season continued when they fell 1-0 at home to Real Mallorca, and Javier Aguirre's cash-strapped side were whistled off at the Romareda.
They are three points adrift at the foot of the standings with 10 points, have lost seven of their last eight games and not scored in the last four.
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