Both players are in typically fine form, with Ronaldo reasserting his worth with a goal in Real's 3-0 win at Liverpool last night to move within a strike of Raul's Champions League scoring record.
Ronaldo has now netted 20 times in 12 games this season, compared to Messi's nine goals in 11 matches for Barca.
However, Ronaldo, who denied Messi a fifth straight Ballon d'Or award in January, has stressed the upcoming match is about the clubs, not the individuals.
"It will be a difficult match because Barcelona are playing well," Ronaldo told his side's website.
"I'm not going to play against Messi, I'm going to play against Barcelona. They are a great team. Real Madrid and Barcelona are playing each other."
Meanwhile, Ronaldo feels the marquee fixture should have been scheduled for Sunday.
"We're not going to make any excuses, but I don't understand why they put the match on Saturday," Ronaldo said.
"It is a difficult match and all footballers know that an extra day's rest is significant. Cup matches aren't so important. However, I don't want to make excuses because the team is fine and confident."