Masur was a semifinalist at the 2012 US Open boys' doubles event, partnering Maximilian Marterer.

He won his first singles match on ATP-level as a qualifier at the 2018 German Open, defeating Maximilian Marterer in the first round.

[1] In the first round of the 2019 Hamburg Open doubles draw, he and partner Julian Lenz upset compatriots Alexander and Mischa Zverev after saving two match points.

Masur qualified for the first time in his career for a Grand Slam main draw at the 2021 Wimbledon Championships.

[3][4] In November, he won the second 2021 Challenger Eckental title defeating Maxime Cressy.