The Reds withstood early pressure from the Basques and could have fallen behind in the stadium where this year’s final will be held on Wednesday 21 May.
But Casemiro headed us into a 30th-minute lead and the momentum truly shifted when Dani Vivian was sent off for hauling Rasmus Hojlund down inside the box.
Bruno Fernandes converted the resulting penalty and stamped his authority on the tie by netting again just before the interval.
Noussair Mazraoui and Casemiro came close to stretching our first-leg advantage further when hitting the woodwork, but the Basques will need something miraculous in Manchester next Thursday if they are to prevent us from returning to this part of northern Spain later in the month.