Real Madrid's response to winning the Clasico was another surprise defeat on Sunday as they lost 2-1 away at Real Betis to hand the initiative straight back to Barcelona.
Betis were deserving victors of a thrilling contest at the Benito Villamarin where two galling Madrid errors cost them their place at the top of La Liga, with Barca now two points clear.
A heavy touch from Sergio Ramos led to Sidnei's stunning opener for Betis and then Karim Benzema, who equalised with a penalty on the stroke of half-time, was at fault for their 82nd-minute winner.
Benzema's misplaced pass in midfield allowed Cristian Tello to go clear and he held his nerve to give Betis their first victory in eight games that adds yet another twist to this season's back-and-forth title race.
"We lacked everything today," said Real coach Zinedine Zidane.
"It was our worst game of the season. We lacked energy, fluency, possession, aggression. It was a bad day and that can happen. I am responsible. We can't lose our heads but we can't be happy either."
Madrid appeared to have taken a significant step forward, at least psychologically, by beating Barcelona 2-0 last weekend but this result undid all their good work at the Santiago Bernabeu while bringing defensive doubts swiftly back to the surface.
Betis climb to 12th and put eight points between themselves and the bottom three.
Madrid have now lost three out of their last four games and won only two from seven, a dip in form that has appeared even more charitable given Barca's own problems on and off the pitch.
After a game at home to Eibar on Friday, Zinedine Zidane's side will travel to Manchester City in the Champions League but on this evidence their hopes of recovering their 2-1 home defeat from the first leg would seem faint at best.
Ramos will be suspended at the Etihad Stadium and his own poor run of form continued against Betis, where he was at fault for their first goal and then complicit in compounding Benzema's error for the second.
"We haven't played the way we trained all week," said Ramos. "We have to be self-critical. And now we have to wait for those above us to slip up."
Eder Militao will likely come in for Ramos against City and he was given an outing at right-back in the absence of the ill Dani Carvajal while Marcelo was preferred to Ferland Mendy on the opposite side.
Gareth Bale was left out of the starting line-up for a fourth match running.
Betis were the better side in the first half and might have taken the lead earlier had Marc Bartra not headed over from four yards or Thibaut Courtois not reacted quickly to push away Nabil Fekir's thundering shot.
But five minutes before half-time they finally broke through, aided by Ramos, who tried to dribble the ball out after Fekir's shot was blocked only for a heavy touch to allow the forward to retrieve.
Off balance, Ramos bundled into Fekir and probably would have conceded a penalty had the loose ball not spilled to Sidnei, who instead hammered it into the top corner.
Deservedly in front, Betis then went in level at the interval as Sidnei, perhaps with the adrenaline still pumping, fouled Marcelo in the box. Benzema drove into the bottom corner for only his third goal this year.
Both teams had chances to find a winner. A scintillating Betis break ended with Sergio Canales sending Joaquin through but after rounding Courtois he tried to square back to Canales when the net looked open. Luka Modric sprinted back and hacked away.
Madrid enjoyed their own spell of pressure as Modric's shot from distance was saved and Mendy, on for Marcelo, curled onto the crossbar, helped on its way by the fingertips of Joel Robles.
But it was Betis that landed the decisive blow, again benefitting from a Madrid mistake. Benzema came deep and his pass inside was loose, persuading Ramos to dash out to intercept but he was beaten and left stranded.
Andres Guardado put Tello through the gap and he sent Courtois diving towards one corner before sliding the ball into the other.