Real Madrid collapsed after leading by two goals to lose 4-2 at Real Sociedad in the Spanish league on Sunday, with the European champions left rudderless without the injured Cristiano Ronaldo.
Lionel Messi, meanwhile, finished Barcelona's 1-0 win at Villarreal with a right leg injury after he set up substitute Sandro Ramirez to score on his debut.
Messi assisted on the 82nd-minute winner and played the full match, but his club said after the game he had injured a right leg muscle and would undergo tests on Monday.
Madrid looked set to romp at Anoeta Stadium after dominating the first half hour with goals by Sergio Ramos and Gareth Bale.
But David Zurutuza scored twice on either side of goals by Inigo Martinez and Carlos Vela as the hosts roared back to leave Madrid's players with blushes to match their bright pink kits.
''We were horrible,'' said Madrid goalkeeper Iker Casillas.
Elsewhere, Isaac Cuenca converted a penalty in stoppage time to salvage Deportivo La Coruna a 2-2 draw against Rayo Vallecano, while David Lomban also scored in added time to secure Elche a 1-1 draw with Granada.
On Thursday, Sociedad was beaten 3-0 at Russian side Krasnodar, eliminating it from the Europa League playoffs and sparking speculation that coach Jagoba Arrasate's job was in danger.
''We began with many doubts after our elimination the other day, but we turned it around with a lot of courage and attitude,'' said Xabi Prieto, who flicked on a corner kick for Martinez's goal and crossed for Zurutuza to score what proved to be the winner.
Now Carlo Ancelotti will have to answer why his squad with stars like Bale, Karim Benzema and James Rodriguez failed to compete for the final hour. Madrid, which had already lost the Spanish Super Cup to Atletico Madrid, slipped three points behind Barcelona.
''I don't like this defeat at all because I think sometimes you can play poorly but when you are winning 2-0 you have to show more character to finish off the game and that is my responsibility,'' said Ancelotti. ''Something has to change and it will change.''
With Ronaldo out to nurse his nagging leg injuries, Ramos and Bale took over the scoring duties.
Ramos headed in Toni Kroos' corner kick in the fifth minute before Bale doubled the lead six minutes later with an audacious dribble to put the ball between the legs