Williams turned an Ibai Gomez cross past Marc-Andre ter Stegen in the third minute of second-half stoppage time to clinch a place in the semifinals with Real Sociedad, Granada, and Cinderella story Mirandes of the Segunda Division.
The Copa del Rey will not include an El Clasico rival for the first time since 2010, when Sevilla beat Atletico Madrid.
Barca was awarded five yellow cards on the day including one to Lionel Messi, who was stopped 1v1 by Unai Simon moments before Williams won it.
Quique Setien’s Barca is three points back of Real Madrid in the race for La Liga, seven points clear of third-place Getafe. The season’s second El Clasico comes March 1 at the Bernabeu.
It said a report of its analysis of the video images of Saturday’s game at RCDE Stadium were sent to authorities.
The club reiterated its “categorical condemnation of any displays of violence and racism.”
The fans made monkey noises directed at Athletic forward Inaki Williams, who is black. Television images showed Williams confronting the section of Espanyol fans who taunted him after he was substituted late in the game in Barcelona.
The Spanish league said it will seek a partial stadium closure for Espanyol and will raise the incident with the Spanish soccer federation’s competition committee, which handles disciplinary issues.
Bilbao’s Williams: ‘I suffered racist insults’ at Espanyol
MADRID (AP) A referee briefly stopped Sunday’s Spanish league match between Sporting Gijon and visiting Athletic Bilbao because of racist chants aimed at Bilbao player Inaki Williams.
Referee Clos Gomez wrote in his match report that he suspended play for a minute in the first half when he heard racist chants imitating what he said were “monkey calls” from one section of El Molinon Stadium directed at Williams.
Gomez says that a warning was then given by the public announcer and that the abusive insults stopped.