Milan warmed up for their UEFA Champions League clash against Manchester United with a 3-1 win over Cagliari.
Ronaldo helped himself to two goals as Milan cemented their place in fourth spot in Serie A.
Carlo Ancelotti rested most of his top stars ahead of Tuesday's trip to Old Trafford, but it did not affect Milan as they broke the deadlock after just 14 minutes through Ronaldo.
The Rossoneri doubled their lead on 69 minutes with Ronaldo converting from close-range.
Cagliari gave themselves hope when Andrea Capone was felled in the box by Kakha Kaladze and David Suazo scored from the resulting penalty.
Milan made the game safe ten minutes from time when Andrea Pirlo came off the bench to score a superb free-kick.
The home side ended the game on a sour note when Kaladze was sent-off in the dying seconds for a second bookable offence.
Sampdoria piled the misery on struggling Messina with a 3-1 win at Stadio Luigi Ferraris.
Reto Ziegler gave Samp the lead on 13 minutes, but Christian Rigano drew Messina level after 54 minutes.
Daniele Franceschini restored Samp's lead seven minutes from time and Gennaro Delvecchio wrapped up the victory with a third goal for the hosts in the final minute.