Barcelona, who won 2-1 at Las Palmas on Saturday, have 63 points to Real's 54 with 13 matches of the season remaining. Atletico Madrid, who play at home to Villarreal later on Sunday, are level on points with city rivals Real.
Ronaldo put Real ahead when he rose unmarked to send a looping header from a Toni Kroos free kick over Carlos Kameni and in under the bar in the 33rd minute for his 22nd league goal of the season.
He was denied a second three minutes later when Kameni dived to his left to save the striker's penalty after he had been brought down by Weligton.
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