Atletico Madrid boosted their La Liga title hopes while simultaneously putting a huge dent in Real Madrid's as Antoine Griezmann's goal settled the derby 1-0.
The city rivals came into Saturday's meeting at the Santiago Bernabeu knowing anything other than victory would represent a significant blow to their title challenge, with leaders Barcelona looking close to unstoppable.
In the event, Griezmann's well-taken goal early in the second half settled the encounter, as Atletico closed the gap to Luis Enrique's side to five points ahead of Barca's home clash with Sevilla on Sunday.
Madrid, however, are nine points adrift in third as coach Zinedine Zidane's hopes of a league crown to go with the one he won as a player in the Spanish capital fade away.
And a section of the Bernabeu faithful made their discontent known, as chants of 'Florentino, resign' rang out, in reference to the club's president Florentino Perez.
Those calls were soon drowned out by whistles as Perez's supporters showed their backing, but the poisonous atmosphere only served to exacerbate another disappointing day for the hosts.
Atleti, meanwhile, retain hopes of a second Liga title in three years as Diego Simeone's side once again displayed their defensive solidity in recording a fourth successive clean sheet in all competitions.
The opening exchanges were frantic but low on goalmouth action, Cristiano Ronaldo and Augusto Fernandez firing harmlessly off-target for either side before Raphael Varane glanced Toni Kroos' free-kick wide.
Fernando Torres - one of three Atletico changes from Wednesday's 0-0 Champions League draw with PSV - was next to threaten as he looked to turn and shoot inside the area but Sergio Ramos was on hand to thwart the former Chelsea striker.
The first shot on target arrived in the 32nd minute as Jan Oblak parried away Ronaldo's thumping free-kick, before Karim Benzema scuffed an effort wide as Real kept the attack alive.
Oblak's opposite number Keylor Navas was similarly impressive shortly before the break, first leaping to tip Griezmann's effort over and then denying a firm strike from Koke at his near post.
Three minutes after the restart, Ronaldo wasted a glorious opportunity to break the deadlock when he fired narrowly wide of the left upright with just Oblak to beat.
And Madrid were left to rue that miss as Atleti went in front five minutes later.
Griezmann found the overlapping Filipe Luis on a counter, and the defender cut the ball back into the path of the France international, who swept a confident finish beyond Navas.
Atletico almost doubled their lead midway through the second half, but Navas was quick to rush from his line and deny Saul Niguez with a sprawling save.
Ronaldo's frustrating outing continued as he headed another golden chance tamely into the arms of Oblak before Borja Mayoral - a half-time substitute for Benzema - failed to make a telling contact as the goalkeeper parried away.
Ronaldo and Danilo both went close in the final 10 minutes, but Atletico's unflappable backline remained solid.