Isaac has scored seven cup goals this season, including two away to Real Madrid, and tops the shooter throughout Copa del Rey. But when Real Sociedad had the chance to secure the final ticket in little Miranda de Ebro, the 20-year-old started on the away team's bench. From there, he saw teammate Mikel Oyarzabal safely put 1-0 on penalty in the 41st minute.
The management goal gave Real Sociedad a huge takeover as the team brought with them 2-1 from the home meeting in San Sebastián.
Alexander Isak came on, Willian José had to leave the fray. After 65 minutes, there was a bit of a mixup on the field. He ran free and chipped the ball into goal in the third overtime, but the goal was sent off for offside.
Real Sociedad has won the Spanish Cup twice, in 1909 and 1987, and has also lost five finals - the most recent in 1988. Thousands of Real fans learn to make the almost 100-mile journey south, to be in place on the long-awaited final in Seville on April 18.
The final could be a hot Basque derby, as arch rival Athletic Bilbao brings with it 1-0 to Thursday's semi-final return away to Granada.