Ronaldo and Messi are the two frontrunners for the 2017 Ballon d'Or, of which the winner will be announced on Thursday.
Should Ronaldo claim the individual gong handed out by France Football, he will go level on five crowns with his Argentinian nemesis.
Despite his underwhelming form in LaLiga this season, having scored just two goals in 10 appearances, Ronaldo is still expected by many to claim the prize, thanks largely down to his role in Madrid's league and Champions League double in 2016-17.
Speaking at a news conference ahead of Wednesday's clash with Borussia Dortmund, Zidane said: "It's true, when we look at their statistics, they're amazing.
"It's surprising to see the amount of goals they've scored, actually. They're two great players.
"One makes the other better and vice-versa. I think it'll continue, this duel, for some time to come."
"I think it [Ronaldo not being at his best] doesn't really matter because he is so big," Zidane said.
"He has big shoulders and when things don't go right for him, he's always found a way to put things right.
"It's normal that people have opinions and talk about him, but people who love Real Madrid, this team, they know what he has done.
"Just last season he had fantastic season. There's a long way to go yet this season, we're not even halfway there – we have six more months to go.
"Be careful with Ronaldo – he'll come good."