Fentz began appearing on the ISU Junior Grand Prix series in the 2008–09 season.

Fentz won his first senior international medal in February 2013, obtaining bronze at the Bavarian Open and then silver at the Hellmut Seibt Memorial.

Ranked 16th in the short and 17th in the free, Fentz finished 16th at the 2016 European Championships in Bratislava, Slovakia.

Due to his result, Germany qualified for a spot in the men's event at the 2018 Winter Olympics in Pyeongchang, South Korea.

Fentz was unable to qualify for the men's event at the 2022 Winter Olympics four years later, but he participated as the German entry in the men's short program of the Olympic team event, where he finished ninth of nine skaters.