Real Madrid moved one point clear at the top of the LaLiga table, following a 2-0 victory over Celta Vigo on Saturday.
With Courtois beaten by a chipped effort from Iago Aspas, Nacho cleared the goal-bound attempt off the line and started a quick Madrid counterattack.
It finished off with Marco Asensio crossing to the far post that was met by a Lucas Vazquez header.
Madrid put the result beyond doubt eight minutes after the break, with Asensio and Vazquez again combining for a goal, this time reversing roles.
Zinedine Zidane's men now have 36 points from 17 games.
However, Los Blancos' position at the top could last little more than 12 hours, with their city rivals Atletico Madrid having three outstanding fixtures and travelling to Alaves on Sunday.