The pair represent both Belgium and Atletico Madrid, with Courtois currently on loan at the Vicente Calderon Stadium from the Premier League side.
Courtois is in his third campaign with the Spanish outfit as Petr Cech continues to be Chelsea's number one.
And Alderweireld has urged Courtois to leave Stamford Bridge permanently in order to fulfil his potential.
"He does not want to sit on the bench for eight games then play for one," the defender told Sky Sports. "He is very smart and I think it would be a good choice for him to maybe stay with Atletico, or go to another club where he will be number one.
"He is good with the ball, he is good with his feet and he is great on his line. I don't think he has a weak point and I think he can be the best keeper in the world.
"I see him in training every day and I see him in games and I doubt there are any keepers better than him."
Courtois was an unused substitute for Belgium in Thursday's 2-0 defeat to Colombia but may return for Tuesday's friendly with Japan.
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