Gorka Guruzeta gave the hosts a first-minute lead but Barca were ahead before the break when Robert Lewandowski equalised and Lamine Yamal made it 2-1

Oihan Sancet levelled before Williams and his brother Nico sealed victory.

Inaki Williams was part of the Ghana squad knocked out of the Africa Cup of Nations on Monday following a draw with Mozambique.

The forward flew from Ivory Coast to Paris before taking a plane to Bilbao and landing around 11am on the day of the match.

He came off the bench in the second half against Barcelona and then found the breakthrough in extra time.

Younger brother Nico, who plays his international football for Spain, scored in the closing moments to secure Athletic Bilbao a semi-final spot.

Xavi's Barcelona side dominated possession, but the host looked the most dangerous and had 27 shots on goal.

However, 16-year-old Yamal, who became the competition's youngest scorer, could have put Barca into the semi-finals in the final minutes but he missed an open goal after rounding keeper Julen Agirrezabala.

Defeat means Barcelona and Real Madrid - who have won 10 of the past 15 Copas del Rey - are both out of the competition.