The Portuguese attacker is still going strong at 34 and the former attacking midfielder is amazed by his motivation to stay on top of the game
Cristiano Ronaldo's motivation to stay at the top of world football comes from his rivalry with Barcelona star Lionel Messi, Kaka says.
Ronaldo and Messi have been regarded as the best players in the world for over a decade, having won five Ballons d'Or each while in competition at Real Madrid and Barcelona.
The Portuguese attacker has maintained his incredible form since leaving Madrid for Juventus, having already scored 24 times in 36 games in all competitions.
At 34, Ronaldo is showing no sign of slowing down and former Madrid and AC Milan hero Kaka believes he is spurred on by his desire to stay ahead of Messi.
"It doesn't surprise me for what it is, it surprises me because he's still at this level," Kaka told La Gazzetta dello Sport of the Juventus star.
"I believe there is also a Messi effect, the infinite challenge, this desire to always outdo himself. At 34 and after so many successes, Cristiano still finds surprising motivations."
Madrid have struggled since Ronaldo left for the Italian champions. They were eliminated from the Champions League last-16 by Ajax, beaten in the Copa del Rey by Barcelona and are nine points adrift of their Catalan rivals in La Liga.
But the recent return of coach Zinedine Zidane is expected to stabilise the Santiago Bernabeu outfit and Kaka is intrigued to see if the Frenchman can replicate his previous success without Ronaldo.
"The cycles end," he added. "Certainly, the result with Ajax is a bit unusual. Now Zidane is back, a great coach who has won three Champions League titles in a row.
"Let's see what he will do without Ronaldo."
Ronaldo is in contention to finish the season as Serie A's top scorer, but he faces competition from Sampdoria star Fabio Quagliarella and 23-year-old star Krzysztof Piatek, who has carried his incredible Genoa form on to AC Milan.