The 32-year-old joined the club from Sevilla in January and has since displaced the Spain keeper and become Madrid's first choice between the posts, having firstly impressed Jose Mourinho last season and then Carlo Ancelotti this term.
Speaking to the club's official website he said: “I've known Iker since I was 18, for about 14 years, and I've always had a good relationship with him.
"It's wonderful to compete in this team with a keeper like Iker. We're both after the spot and we both give everything we have to play.”
“It's the manager that decides the rotation. He decides and we have to try to do our best with it. The most important thing is to be ready every day and train your hardest.”
Another hat-trick from Cristiano Ronaldo helped Madrid to a 5-1 win over Real Sociedad on Saturday and Lopez believes the squad are beginning to adapt to their new coach's philosophy.
“We knew that adopting Ancelotti's ideas was only going to be a question of time," he added.
"Little by little we're seeing the team that the boss wants and today, especially in the first half, we've taken another step towards what he's after.
“Ronaldo is incredible. Today we saw, once again, what a unique player he is. We're very lucky to have him here.”
Lopez has made 14 appearances so far this season, with Casillas featuring in all four of los Blancos' Champions League fixtures.