Berenguer's late strike sends Real Madrid crashing out of Copa del Rey
Athletic Club booked themselves a Copa del Rey semi-final place alongside Real Betis, Rayo Vallecano and Valencia with a late victory over Real Madrid.
Alex Berenguer was the hero as he turned Nacho inside out on the edge of the box before curling a shot into the far corner beyond Thibaut Courtois on 88 minutes.
The late strike sent a raucous San Mames wild and was enough to see out the win for Athletic Club, who got revenge for the Supercopa de Espana defeat against Los Blancos only a few weeks ago.
The game itself was one of few chances for either side, with neither quite prepared to gamble and risk opening themselves up to their opposition.
Real Madrid's greatest threat came through Vinicius Junior, fresh from having played for Brazil only 48 hours before, and he was rested from the hour mark as he was replaced by Isco.
Athletic Club had their own energetic forwards bombing down the wing, and their best chance came as teenager Nico Williams led a counter-attack, but his fumbled pass saw the move breakdown with Athletic outnumbering Real Madrid's defenders.
There were also penalty shouts, with the most controversial involving Nacho as he rose to clear a corner, with the ball seeming to impact his arm as he rose in the air, but VAR determined that there was no foul.
Carlo Ancelotti had forward Luka Jovic to call upon in the absence of Karim Benzema due to injury, but he was surprisingly omitted from the line-up and was not brought on from the bench either.
The Italian left the Serbian, Gareth Bale and Eden Hazard on the sidelines while a starting attacking of Marco Asensio, Rodrygo Goes and Vinicius managed only two shots on target.
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.