[1] In 2010, Mager was disqualified for failing a doping test after smoking marijuana at a party.

He got his first singles ATP main-draw win in the first round in Stockholm in October 2019 against Spaniard Pablo Andújar.

At the 2020 Rio Open in Brazil, an ATP 500 level tournament on clay courts, he beat world No.

[4] Mager reached his first ATP Tour final, defeating lucky loser Attila Balázs in the semifinals before losing to Chilean Cristian Garín, 6–73–7, 5–7.

At the 2021 Sofia Open Mager reached his fourth quarterfinal of the season at the ATP 250 level (after Delray Beach, Belgrade and Kitzbühel) defeating sixth seed Adrian Mannarino and Miomir Kecmanović.