Along with Manchester United's David de Gea, the Atletico Madrid goalkeeper is one of the most promising in his position, according to the Spain No.1
Casillas and Courtois will be between the sticks in Saturday's Champions League final, when Real and Atletico Madrid go head-to-head for European football's ultimate club prize.
The Spain veteran has nothing but respect for his Atletico counterpart, and insisted that he is no longer a promising keeper, but is already at the top level.
"Courtois is a fantastic goalkeeper who has really improved," the Spain keeper told reporters. "He is no longer a promise - he is a reality. With David de Gea, these are the two goalkeepers of the future."
The 33-year-old, who was in goal when the Spanish giant last won the Champions League 12 years ago, feels his side deserves to be in the final and has learned a lot from recent matches, as it ended the league season with just one win in four games, surrendering the title to Saturday's opponent and finishing behind Barcelona.
"In recent days we haven’t been talking about the final. At last we have made it again, we want to win, because of everything we have done since we started the season.
"We deserve to be in the final. In the past we have learned from some games where we underestimated our opponents, and that has helped us this year to take things one game at a time. It took us time to get going this season but we were in contention at the end.
"We have to aspire to win everything. Unfortunately we didn't manage to get the Liga title so now we have to think about this final. We know Atletico's strengths off the ball and on set pieces. Any small detail can change a game like this.
Reflecting on his previous experience in a Champions League final back in 2002, he said: "When you are 19 you don't realise just how much a match like this means. After time you see what something like this means to a club like Real Madrid and you see it is immense. I've played in big finals for Madrid and Spain and you see the privilege you have to make millions of people happy, thousands travelling by car, you want to send them home happy."