Real Madrid fell to lowly Rayo Vallecano 1-0 at the Campo de Futbol de Vallecas on Sunday, leaving manager Zinedine Zidane to hit out at his team's lacklustre display after the game.
The toothless display comes a day after the Bernabeu side -- who remain in third place after the result -- saw eternal rivals Barcelona celebrate their 26th La Liga title, and second straight, with a win over Alaves on Saturday.
The only goal came when Adri Embarba sent Real goalkeeper Thibaut Courtois the wrong way with a penalty midway through the first haalf, giving Rayo a deserved lead after an ambitious start in front of a typically raucous home crowd.
The spot kick, given for a foul by Jesus Vallejo on Javi Guerra, was awarded following a VAR review after the referee had waved play on and Gareth Bale had narrowly failed to score as Madrid counter-attacked.
Real later had a goal from Mariano ruled out for a clear offside, one of the rare occasions in the game in which Zidane's side troubled the hosts, who looked far likelier to score again than concede a goal to Madrid's toothless attack.
The loss kept Madrid on 65 points with three matches left to play.
It was Rayo's first win over their illustrious neighbours since 1997, when Fabio Capello was in charge of Madrid.
"Today we did nothing, from minute one until the end, at all levels," Zidane said after the match. "Sometimes you cannot score, but today we did not even make chances, did not do anything in our play. We must be very angry, and I am angry, as our image today was bad. And I am responsible for that, not just the players. We must apologise for what we did today, all of us."
Zidane was asked directly was he sorry to have returned to the Bernabeu to take over a Madrid side which had slumped through the season under previous coaches Julen Lopetegui and Santi Solari.
"I am not sorry, just the opposite," he said. "I have to live through these difficult moments like today. But we cannot play like this. Today it was everything, the attitude, everything. We were not committed in anything, challenges, running. They played their game, but we did nothing of what we must do on the pitch."
Zidane would not reveal what he had told his players after the game.
"What I said to the players stays there. I will always defend my players, but not today. That is the reality of today, we cannot play like this. But it is not just them, I am responsible as I pick the team and prepare the game with the players. Today was just opposite of what was planned."
Meanwhile, the win moved Rayo above Huesca to 19th on 31 points. They are still six away from escaping the relegation zone, but their supporters showed they believe they can still avoid the drop, chanting "Yes we can" when the final whistle blew.
The build up to the game was dominated by Real's refusal to allow Rayo's top scorer Raul de Tomas, who is on loan from Zidane's side and prevented from facing his parent club due to a clause in his contract, to turn out for the home side.
Reuters and ESPN FC Madrid correspondent Dermot Corrigan contributed to this report.