Grassl placed sixth in the short program at the 2020 World Junior Championships in Tallinn, Estonia. Grassl attempted the quad flip in competition for the first time in the free skate, underrotating both it and his quad Lutz, as well as a triple Axel. He placed third in the segment, winning a small bronze medal, and was fourth overall, 1.74 points behind bronze medalist Petr Gumennik. Grassl was scheduled to make his senior World Championship debut in Montreal, but these were cancelled as a result of the coronavirus pandemic.