King's Cup holders Barcelona have a fight on their hands if they are to remain in the competition after losing a fiery first leg of their last-16 tie at Athletic Club 2-1 on Thursday despite playing the final 10 minutes with a two-man advantage.
Spain striker Aritz Aduriz headed Bilbao in front in the 25th minute and the veteran produced an audacious flick to tee up youngster Inaki Williams to volley in the second three minutes later.
Talisman Lionel Messi pulled a full-strength Barca side back into the game with a free-kick in the 52nd minute which came off the underside of the bar and over the line.
Brazilian forward Neymar had a strong penalty appeal turned down in the first half and was the victim of more aggression after the break, leading to Raul Garcia and then Ander Iturraspe being shown second yellow cards late in the game.
Roared on by the famously passionate San Mames faithful, Athletic produced a resilient display to hold on for the win despite a barrage of late attempts by Barca, which included Messi hitting the post with the last kick of the match.
The second leg takes place next Wednesday at the Nou Camp.