The Spanish side won the second leg of their last-16 tie against AC Milan 4-1 at the Vicente Calderon on Tuesday to wrap up a 5-1 aggregate success.
It is first time in 17 years that the Rojiblancos will feature in the last eight of Europe's premier club competition, and their 21-year-old, who is on loan from Chelsea, is overjoyed with his side's progress
"It is nice," Courtois told Sky Sports 1. "We worked very hard for it. The first year (coach) Diego Simeone came we just missed the Champions League.
"Last year we played well and finished third in La Liga, and this year we are playing in the Champions League.
"The two games against AC Milan were very good, maybe at the end of the first half (today) it was a bit difficult. But I think overall we played a very good game."
Courtois also praised his Atletico team-mate Diego Costa, who scored twice against Milan in Madrid.
"He is perfect," said the Belgium international. "From one or two chances he scores and that is really important for us. He scored two goals, so we can be very happy."