Klassen made his professional debut in the 3. Liga for 1860 Munich on 31 July 2019, coming on as a substitute in the 87th minute for Benjamin Kindsvater against FSV Zwickau. He assisted Herbert Paul for Munich's third goal, with the match finishing as a 3–0 home win.[3]
On 27 May 2021, he signed with WSG Tirol in Austria.[4]
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