The Portugal star has praised his team-mate for his consistency this term and claims his enjoyment of the game is what keeps him motivated
The Croatia international struggled for form initially following his move from Tottenham in the summer of 2012, but has become an integral part of Carlo Ancelotti's new 4-3-3 system at Santiago Bernabeu in 2013-14.
Ronaldo, who faced Modric on a handful of occasions during their respective spells in the Premier League, feels the 28-year-old deserves enormous credit for his consistently strong displays for Madrid.
"I've known him for many years from the Premier League. He's showing the best football his my career this season. Not only playing fantastically, but to do so continuously, in Spain, for this club is not easy," he is quoted as saying by Snplus.
"He is one of our key players. He is a modest and humble guy. We get on really well."
Ronaldo is in line to return to action against Bayern Munich this week after coming through training unscathed over the weekend and the 29-year-old admits the enjoyment he gets from playing means he never likes to be sidelined.
"I always want to play. I enjoy it because football is my work and my life's passion," he said.
"More than motivation, it's the enjoyment I get from every game that means it's not so hard to try to be at my best.
"I always try to improve, to achieve my goals and not be defeated. It's not easy to be at this level consistently, but with strength and passion I give my best every day."
Ronaldo is set to lead Portugal in the World Cup this summer and while he admits his side cannot be considered among the favourites in Brazil, he insists they can lay down a marker in their opening clash with Germany.
He continued: "Portugal are not among the favourites, such as Spain, Germany, Argentina. We have no pressure and will concentrate on each game as a path to success. The first game against Germany will indicate what our chances are."