The goalkeeper reveals another side to his relationship with the Portuguese forward, but has hailed his teammate's impressive goalscoring exploits this season
Reports of a rift were rife earlier on this season and while the two have both denied any animosity and have even been pictured together - with their girlfriends - at social events, Casillas says their relationship is not always harmonious.
In an interview with ESPN, the goalkeeper said of Ronaldo: "He's a friend and a teammate. A teammate and a friend. But he's also an enemy, because we argue a lot."
Ronaldo hit 40 league goals for Madrid this term (53 in all) and Casillas hailed the 26-year-old's scoring contribution, saying he would pick the Portuguese ahead of five-time Pichichi winner Hugo Sanchez in a Real Madrid dream team.
"I never played with Hugo and I did not see him play much," the goalkeeper said.
"I will stick with what I know and that's Cristiano. It's not by chance that he scored 40 in a season. And he has done it before so he must be good."
Casillas also praised coach Jose Mourinho for making the cub competitive again.
"He [Mourinho] has formed a competitive team with aggressive character and he has made us more mature," he said.
"We are a young team and our first tooth came out with the Copa del Rey. But we are not satisfied and we have to be more ambitious."
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