Athletic Club booked a Copa del Rey semi-final place with a late strike from Alex Berenguer shot into the far corner beyond Thibaut Courtois on 88-minute to dash Real Madrid's hopes of clinching a Spanish domestic double.
The late strike was enough to knock Madrid out of the Copa del Rey, Athletic Club booked themselves a Copa del Rey semi-final place alongside Real Betis, Rayo Vallecano and Valencia, and also a revenge for the Supercopa de Espana defeat against Los Blancos only a few weeks ago.
With Barcelona, Atletico Madrid and Sevilla already out, Madrid were firm favourites to win the Copa del Rey while in La Liga, they are in the driving seat too, sitting four points ahead of Sevilla at the top of the table.
The match was intense and compelling but low on chances, with neither team able to carve out any golden opportunities, even if Athletic were the more aggressive overall.
When the decisive moment came, it came from a mistake as Casemiro's sloppy pass was cut out by Mikel Vesga, who quickly fed into Berengue who turned Nacho inside out on the edge of the box before curling a shot.
None of the top four from LaLiga Santander last season have made the semi-finals of the Copa del Rey, with Athletic Club set to compete for a possible third consecutive final appearance.
Friday's draw will pit them up against Real Betis, Rayo Vallecano or Valencia with a two-legged affair separating them from a place in the final at Estadio La Cartuja in Seville in April