Gareth Bale scored his first Real Madrid goal since September to help his side beat third tier Unionistas and reach the last 16 of the Copa del Rey.
The Wales forward scored a deflected opener after 18 minutes in Salamanca.
Unionistas substitute Alvaro Romero levelled with a fine left-footed strike into the top corner after the break.
But Juan Gongora’s own goal put Real back in front before former Manchester City midfielder Brahim Diaz made sure of victory in added time.
Bale, 30, has been affected by a calf injury this season and there has been ongoing speculation that his relationship with manager Zinedine Zidane is strained.
His last strike for Real was a double against Villarreal in La Liga on 1 September.