The 35-year-old is one of the most successful footballers ever with his haul of five Ballon d'Or awards bettered only by Lionel Messi, while his tally of five Champions League titles is the most of any player in its present era.
However, speaking to two-time middleweight world boxing champion Gennady Golovkin in the documentary "Parallel Worlds" for DAZN, he said he prefers combat sports.
"Playing football is my passion, but I prefer watching other sports on TV," Ronaldo said. "Between watching a football match or a boxing or UFC fight, I choose boxing or UFC."
Ronaldo added that his passion for boxing started while playing in England.
"When I was at Manchester United, a coach boxed with me," Ronaldo added. "I think practicing boxing is useful for football because it sharpens your senses and you learn to move."
Despite being in the twilight of his career, Ronaldo has kept on producing incredible goal scoring campaigns and hit 31 in Serie A last season.
He said that he is determined to adapt and not lose the physical attributes which made into one of the greatest footballers of all time -- and used similar examples of other sports to explain how it is possible to continue performing later in life.
"Last summer, I had a chat with Anthony Joshua," Ronaldo said. "At 33 you start to think your legs are going.
"I want to stay in sport, in football. People will look at me and say: 'Cristiano was an incredible player but now he's slow.' I don't want that.
"You can change a lot about your body, but the problem isn't that. It's depends on your mindset, your motivation and your experience, which I think is the most complex thing.
"In sport, you can gain maturity. Look at [Roger] Federer in tennis. He's 37 or 38 years old and he's still at his peak, and there are some in boxing too."