The Portuguese has 15 netted times in the first eight league games and Real move up to second in the table, a point behind leaders Barcelona who they meet next weekend.
Ronaldo slotted home a penalty after 13 minutes following a foul by Juanfran on Javier Hernandez and the Mexican striker headed in from close range before halftime.
After the break Ronaldo completed his brace with a run down the left wing and a clinical finish. James Rodriguez volleyed in the fourth after 65 minutes and Isco completed the rout with an 18-metre drive.
“To score more than Ronaldo is doing would be impossible. He needs to continue with this form and we will help him to do so,” Real coach Carlo Ancelotti told a news conference.*
“We are on a good run but we need to remember that behind the goals there is sacrifice, fight and concentration. You don’t score without sacrifice and concentration. The best players realise that.
“We are arriving in good condition (for the El Clasico). It was a difficult game to prepare us and we looked solid, full of concentration and we got a good result.”
Lionel Messi, who celebrated 10 years since his Barcelona debut this week, has his sights set on Telmo Zarra’s La Liga scoring record when the Catalan side entertain Eibar later.
The Argentine is two behind the 251 scored by Zarra for Athletic Bilbao during the 1940s and 1950s.
Champions Atletico Madrid, down in fifth place with 14 points from seven games, face Espanyol on Sunday.