Madrid were next to go close after 18 minutes via Gonzalo Higuain, but the Argentine could only fire high and wide when he might have done better.
The away side began to dominate proceedings as the half wore on and twice Cristiano Ronaldo went close to finding the bottom corner.
The best chance of the half fell to the home side after 39 minutes, however, when Piti rattled the post after good work from Armenteros.
Madrid took the lead just ten minutes into the second period when Ronaldo struck a low drive into the bottom corner from the centre of the box.
Ronaldo again went close after 63 minutes with an effort from outside the area, but Joel was on hand to make a smart save at his near post.
The home side pushed for the equaliser in the latter stages, despite losing Michu to a red card, and came close via an effort from Tito, but Madrid held on to further extend their lead at the top of La Liga.