High-flying Valencia and Malaga failed to win in Primera Liga on Sunday, while Real Zaragoza secured a vital victory at the bottom.
Espanyol recorded their first win in six games in some style as two goals in either half saw them trounce third-placed Valencia 4-0 at Cornella-El Prat.
Cristian Gomez opened the scoring with a neat finish from Sergio Garcia's cut-back and Joan Verdu added the second four minutes later with an imperious strike from outside the box.
Alvaro Vasquez came off the bench to slot home from another Garcia assist just before the hour mark and Kalu Uche rounded off a perfect afternoon for Mauricio Pochettino's side when he finished Victor Sanchez's fine through ball 10 minutes from time.
Victory moves Espanyol up to 45 points, just one off Osasuna in sixth, whilst Valencia are now just a point ahead of Malaga in the battle for UEFA Champions League football next season.
Fourth-placed Malaga had to settle for a 1-1 draw at La Rosaleda after Claudio Bravo pulled off a string of wonderful saves to salvage a point for 10-man Real Sociedad.
Malaga went in front after 19 minutes as Isco's shot from outside the area took the slightest of deflections off Ion Ansotegi and left Bravo with no chance.
The hosts seemed set to move back into third place when Mikel Gonzalez was dismissed for a second bookable offence just before half-time, but Sociedad were level three minutes after the break as Xabi Prieto's shot from distance found the bottom-left corner of the net.
Malaga then threw everything at the Sociedad goal but found Bravo in unbeatable form as he saved brilliantly from Jesus Gamez, turned Salomon Rondon's overhead kick onto the post and then again denied the Venezuelan with a stop from point-blank range three minutes from time.
Athletic Bilbao warmed up for their Europa League semi-final in midweek with a 1-0 win over Real Mallorca thanks to Fernando Llorente's 28th goal of the season.
The Spanish international headed the hosts in front after just 12 minutes as he got on the end of Ibai Gomez's free-kick, but goalkeeper Dudu Aouate will have been disappointed to see the ball slip between his legs and into the net.
Mallorca were unfortunate not to be level before half-time as Pedro Bigas' shot came back off the post, but they did not seriously threaten an equaliser in the second half as Athletic held on to move within six points of Malaga in the final UEFA Champions League place.
Real Betis took another step to ensuring they will definitely be playing top flight football next season as Ruben Castro's goal was enough to hand them a 1-0 win over Osasuna.
Castro's fine strike from just outside the area 10 minutes before half-time was a rare moment of quality in an otherwise uninspiring first half.
And any hopes of an Osasuna comeback after the break were dealt a blow on the hour mark when top-scorer Raul Garcia was given an extremely harsh red card for a foul on Jefferson Montero on the halfway line.
Lautaro Acosta's last-gasp equaliser secured a point for rock-bottom Racing Santander and put a dent in Villarreal's bid to pull clear of the relegation zone after a 1-1 draw.
The home side, desperate for some good news after losing star striker Giuseppe Rossi for another six months this week due to a recurrence of a knee injury, appeared to be heading for all the points after Hernan Perez put them in front early in the second half.
But in the fourth minute of injury time Acosta got his head onto Jairo's cross to secure a dramatic draw for the visitors.
The point does little to ease Racing's plight at the foot of the standings, and they remain 10 points behind Villarreal.
But Villarreal will be frustrated to see their gap to the drop zone cut to five points, after Real Zaragoza, second-bottom at the start of the day, closed the deficit with a 1-0 win over Granada.
Tomislav Dujmovic struck in only the sixth minute as Zaragoza moved into 18th place, one behind their opponents who are still in grave danger of being sucked into the relegation zone with five games left.
Zaragoza finished the match with 10 men following Ruben Micael's stoppage-time dismissal, but it came too late for Granada to take advantage.
The final game of the day saw Atletico Madrid claim a 1-0 win at Rayo Vallecano courtesy of Radaem Falcao's goal just after the hour.