Former Portugal midfielder Deco has claimed that Cristiano Ronaldo's dedication to becoming the best player in world football borders on 'a sickness'.

Deco played alongside Ronaldo for the Portuguese national team between 2003 and 2010 and has said that he has 'never seen anyone' put as much into training sessions as the Real Madrid attacker.

The former Chelsea and Barcelona midfielder has also claimed that it is impossible to compare Ronaldo with another of his former teammates, Barcelona's Lionel Messi.

"The guy is sick, I haven't seen anyone in training sessions like that. I think it's really difficult to be like that," Deco told ESPN Brasil.

"They are completely different. Though Messi takes care of himself like a normal athlete, Ronaldo is unbelievable. It borders on sickness because he always wants to be the best at his job. He competes in everything.

"I think it's great to see both players in action, it's almost impossible to compare them. If you look at one's game it's totally different from the other's. There are no parameters for comparison."

Deco retired from professional football in 2013 after leaving Fluminense.