Bilbao held by Gijon after penalty miss
The Guinean forward evaded two defenders chasing a long ball over the top to score midway through the second half.
Financially-troubled Mallorca's cup run is a welcome distraction from the Primera Liga, where they are second bottom without a win since late October.
Earlier, Athletic Bilbao's Fernando Llorente (pictured) missed a first-half penalty as his side were held to a 0-0 home draw by Sporting Gijon in the first leg.
The lanky striker had the perfect chance to break the deadlock after going down to win a dubious spot kick but he blazed his effort high and wide.
Bilbao were put under constant pressure by the visitors in an open game but the 23-times Cup winners held firm to extend their unbeaten run to 10 games in all competitions.
On Wednesday, holders Valencia beat 2007 winners Sevilla 3-2 at the Mestalla and Espanyol held city rivals Barcelona 0-0 at home. The second legs are next week.
Espanyol or Barca will play Mallorca or Betis for a place in May's final and Valencia or Sevilla will take on Bilbao or Sporting.