Hertha chief Arne Friedrich has revealed that his former Germany international team-mate Mertesacker pointed him in the direction of Guendouzi in the summer.
"Every day he shows in practice what a good player he is.
"He's so calm. He wants every single ball, even under pressure. He has his own understanding that he wants to win games.
"He has this intrinsic motivation to just show the world how good he is. This is what we need, this is what everybody needs and he is highly confident, which is also very important for our team because our team is still looking for structure.
"We are very, very happy to have him here and he's worth every penny."