© Getty Images
The home side were ahead inside the opening five minutes, as Angel di Maria slid the ball in for Ronaldo, who converted from close range.
Carlos Tevez had a good chance to level the scores, but the Argentine smashed his effort high and wide.
The La Liga side were able to edge ahead again just six minutes later when Sergio Ramos was brought down in the box, giving Ronaldo a chance to add his second from the penalty spot.
Andrea Pirlo blasted the ball over, while Casillas stopped another effort from Tevez before the end of the first half, but the Italians could not get back on level terms before the break.