Iker Muniain struck in extra-time in seal the stunning 3-2 win.
“A Copa night at San Mamés is always special and tonight it became something magical," Marcelino said at his post-match press conference.
“I don't think I'll forget it for as long as I live. We were the better side for most of the game against a great opponent. Football has been fair to Athletic today and all of this was enhance by the fans. You can't imagine what they sounded like.
“We scored a goal right at the start, they equalised, but we never doubted ourselves. I think we should have gone in at half-time with a lead.
“We took the lead again and, when we were close to victory, the game went to extra time, but we got back on our feet. All the statistics except possession tells us that we were superior and that we deserved to win."
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