Young Real Madrid striker Gonzalo Garcia headed home a 93rd minute winner to put his side into the Copa del Rey semifinals with a 3-2 victory at Leganes on Wednesday.
Juan Cruz's brace for the hosts pulled back early strikes by Luka Modric and Endrick for the European champions at Leganes' Butarque stadium.
With extra-time beckoning 20-year-old forward Gonzalo headed home at the near post after fine work by Brahim Diaz to help Madrid snatch a derby victory and keep their legs fresh ahead of big upcoming games against Atletico Madrid and Manchester City.
Madrid were without Kylian Mbappe and Jude Bellingham after minor knocks, and started with youngsters Jacobo Ramon and Raul Asencio at the heart of their back-line because of their defensive injury crisis.
Madrid started well and were soon ahead, with Modric slotting home in the 18th minute after good work by in-form winger Rodrygo.
Teenage striker Endrick doubled Los Blancos' lead with a poacher's finish after the ball broke loose to him in the box.
It seemed to be plain sailing for Carlo Ancelotti's side until Jacobo conceded a penalty for handling Cruz's shot.
The Leganes playmaker dispatched the spot kick low to Andriy Lunin's left, with the Ukrainian goalkeeper inches away from reaching it.
Ancelotti sent on Vinicius Junior for Rodrygo for the second half, and the winger missed two good chances straight away.
Leganes goalkeeper Juan Soriano saved well to deny the Brazilian, who also headed over from close range.
Madrid paid the price for their profligacy when Cruz netted his second, with the aid of a deflection off Ferland Mendy.
With stoppage time almost over, Diaz produced a fine dribble to break free on the right, before crossing to the near post to create his side's late winner.
Gonzalo was on hand to head home with one of the final touches of the game and book Madrid's ticket to the semifinal.
Atletico thrashed Getafe 5-0 on Tuesday to progress, while record 31-time winners Barcelona visit Valencia on Thursday, and Real Sociedad host Osasuna.