A 10-man Real Madrid faltered once again in the LaLiga title race as Takefusa Kubo haunted his former club after scoring in a 2-0 win for Real Sociedad.
Barcelona snatched a 1-0 victory over Osasuna prior to Tuesday's kick-off at Reale Arena, where Madrid offered little without the suspended Vinicius Junior or the rested Karim Benzema.
Kubo – who joined La Real from Los Blancos last July – scored just past half-time and, after Dani Carvajal received his marching orders for two cautions, Ander Barrenetxea made sure of victory for the hosts.
Next week's Champions League semi-final clash at Manchester City appears the priority for Madrid now over LaLiga, where they trail leaders Barca by 14 points with five games to play.
Alex Remiro made an expert stop to deny Eder Militao with a sixth-minute header bound for the top-right corner before La Real steadied their early nerves.
Martin Zubimendi turned Alexander Sorloth's near-post flick-on onto the crossbar from point-blank range, while Thibaut Courtois repelled a glorious David Silva chance with his feet.
Mikel Oyarzabal saw a penalty appeal for a foul by Militao rejected as Sociedad continued in the ascendancy, albeit without a first-half reward arriving.
Yet their dominance told immediately after the interval when Kubo capitalised on a stray Militao pass back to Courtois and poked into an empty net.
Matters worsened for Madrid as Carvajal was sent off for a second needless lunge on Aihen Munoz, before Barrenetxea sealed the deal in the 85th minute when he smashed a low finish into the bottom-left corner.
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La Real coach Imanol Alguacil has taken more points off Madrid in LaLiga than any other manager in the competition's history, moving past Paco Lopez (10) after making it 12 in 10 games (W3 D3 L4).
While the visitors were without star winger Vinicius, Los Blancos had won all three fixtures this season when he had not featured and avoided conceding in those outings.
Then there was Carlo Ancelotti's decision to rest Benzema, displaying the Italian's obvious priority – Alguacil may have Madrid's number but Los Blancos' calling card remains firmly with the Champions League.
King Kubo
Loan spells at Mallorca, Villarreal and Getafe marked an unsuccessful spell in Kubo's Madrid career, with Los Blancos showing little interest in keeping him.
But the La Real forward made his former side pay here, scoring his eighth goal in 30 LaLiga games for Alguacil's side – two more than in his previous four seasons combined (six goals in 94 outings).
More needed from Militao
Miliato went close in the opening stages to scoring his sixth headed goal this season in Europe's top-five leagues – no player can match his current mark of five either.
Yet the Brazil centre-back's momentary lapse in concentration in the second period, crumbling under the pressure of Sorloth before rolling in Kubo, will be what this performance is remembered for.
What's next?
Madrid face Osasuna in the Copa del Rey final on Saturday, a week before La Real host Girona in LaLiga.