Lopez, 32, has ousted Casillas as first choice at Madrid since arriving in January, with the former Sevilla keeper claiming that he is relishing the battle.
"I've known Iker since I was 18, for about 14 years, and I've always had a good relationship with him," he told the club's official website.
"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."
Casillas has been widely linked with a move away from Madrid in recent months.