Thijssen won the bronze medal at the 2015 UCI Juniors Track World Championships in the points race. He won the silver medal at the 2016–17 UCI Track Cycling World Cup in Apeldoorn in the team pursuit. In 2016, as a junior he won 2 stages in the Nations Cup Trofeo Karlsberg, the Omloop van de Vlaamse Gewesten and a Belgian Cup stage in Nederhasselt. He represented Belgium at the UCI Juniors Road World Championships in Qatar. In 2017, as a U23-rider he won the Grote Prijs Stad Sint-Niklaas and a stage in Olympia's Tour. He won the gold medal at the 2017 UEC European Track Championships in the Elite Elimination Race. In 2018, he was the Belgian champion in the U23-category. In November 2019, Thijssen crashed out of the Six Days of Ghent, suffering three small brain haemorrhages before being admitted to intensive care.