Cristiano Ronaldo's "perfect" work ethic at Real Madrid is the reason why he has been able to stay at the top of the game for long, says former Atletico Madrid star Simao.

Madrid face off against their city and LaLiga rivals in the semi-finals of the Champions League, with Los Blancos aiming to retain the title they won by beating Atleti in a penalty shootout last season.

Simao's loyalties may be with the red and white side of Madrid, but he paid tribute to his former Portugal team-mate's appetite and desire to constantly improve ahead of Tuesday's first leg at Santiago Bernabeu.

"Every day, when he wakes up, he is the first one on the bench," Simao told Omnisport. "He starts to work one hour before the training starts. He trains and after the training, he keeps working. That is in that level that nobody expected.

"He always wants more. He knows that if he wants to be at this level, he needs to work more and more. He has been always a winner, who when he wakes up he just thinks of work and work.

"At a professional level, he is perfect, he just thinks of work, rest, eat well and compete. That is why he keeps his shape. The most difficult thing in football is to maintain that level. You can be on the top, but to stay there as many years as Cristiano has, it is so difficult.

"He always scored a lot of goals. If you compare him with [Barcelona's Lionel] Messi, he improved his scoring rate, to know which position he should be to score. He is looking more for a central area, to score, because before, on the sides, it was more open.

"But right now, he is playing between the lines, and he is playing as a striker, with more options to score. He lost the one on one, that was incredible, but he gained the opportunity to score more goals. He lost one thing but gained another one - and in football, the most important thing is the goals."

Despite his admiration of Ronaldo, Simao believes the way Atletico are prepared for matches by coach Diego Simeone gives them a chance against Madrid, who have beaten them in the final two seasons out of the last three.

"The big change I noticed [with Simeone] it is that the players are motivated from the warm-up," Simao added. "And that is important. 

"They are always ready and it is essential, because you cannot waste five minutes in a game. So they are motivated from the warm-up. It is the way they are on the pitch and the way they work together. 

"They know exactly what they want and what they have to do and not to do, they know their limits. And this is essential in a team."