Real Sociedad beat second division Mirandes 2-1 at home in the Copa del Rey semi-final first leg on Thursday to stay on course to reach the final for the first time in 32 years.
Spain international Mikel Oyarzabal fired the home side into the lead in the ninth minute at a heaving and noisy Reale Arena.
But Mirandes, who also reached the semi-finals in 2012 and have knocked out three La Liga sides this season, levelled in the 40th through Brazilian forward Matheus Rodrigues.
The striker, on loan from Premier League Watford, controlled a pass into the area and fended off three defenders before sending the ball tricking into the bottom corner to score a fifth goal in four Cup games this season.
Sociedad's Norway midfielder Martin Odegaard lost the ball in the build-up to the equaliser but redeemed himself three minutes later by netting on the rebound to restore the lead.
The Basque side had more chances after the break but were kept at bay by keeper Limones, who gave Mirandes hope for the second leg at their tiny Estadio Municipal de Anduva where they have knocked out Celta Vigo, Sevilla and Villarreal this season.
"We created a lot chances and were hoping for a better result but we have to look on the positive side and try to finish the job at their ground," Oyarzabal told reporters.
Sociedad last won the Cup in 1987 and are bidding to reach the final for the first time since 1988.
They pulled off a stunning 4-3 away win over Real Madrid in the quarter-finals and their team bus was greeted on arrival at the stadium by thousands of supporters waving flags and scarves, some of whom lit blue flares.
Sociedad's local rivals Athletic Bilbao beat Granada 1-0 at home on Wednesday in the other semi-final, setting up the prospect of an all-Basque final in Seville on April 18.