Athletic Bilbao fought back from a goal down to defeat Atletico Madrid 2-1 in the Spanish Super Cup final, setting up a showdown with Real Madrid.
Just after the hour mark, the Basque team fell behind after Joao Felix’s header hit the post before being redirected into the goal off the back of goalkeeper Unai Simon.
With a 3-2 triumph over Barcelona on Wednesday, Real Madrid ensured their spot in Sunday’s final in Riyadh.
Williams outran Atletico Madrid captain Koke to head the game-winning goal past Jan Oblak in goal.
“I’m very emotional right now,” said the 19-year-old, who lined up for the match alongside older brother Inaki.
“I hope there will be many more joys like this. I’m very happy to have been able to experience this with my Inaki and my mother, who came to hug us at the end of the match.