Grassl began learning to skate in 2009. His first coach was Ludmila Mladenova in Merano. He competed internationally in the advanced novice ranks from the 2012–2013 season through 2014–2015. Making his junior international debut, Grassl won the bronze medal at the Lombardia Trophyin September 2015. He won his first Italian national junior title in December of the same year.
Coached by Magri, Grassl began his season on the JGP series, placing 6th in Brisbane, Australia, and then 7th in Egna, Italy. His senior international debut came in late October 2017, at the Golden Bear of Zagreb in Croatia; ranked first in both segments, Grassl outscored British champion Graham Newberry by 13.33 points for the gold medal. During his time in Croatia, he was the youngest skater ever to land a quad lutz. In November, he stepped onto two more senior international podiums, taking gold at the Ice Challenge in Austria and then bronze at the Merano Cup in Italy.
2018–2019 season
In August, Grassl won bronze at the ISU Junior Grand Prix in Slovakia. In October, he received the senior gold medal at the Golden Bear of Zagreb after becoming the first European to land a quad loop in international competition. In December, he outscored Matteo Rizzo by 4.48 points to become the Italian national senior champion. Grassl next competed at his first European Championships, where he placed ninth in the short program and fifth in the free skate, for sixth place overall. As Rizzo won the bronze medal at Europeans, he was assigned to Italy's lone men's place at the 2019 World Championships, while Grassl was sent to the 2019 World Junior Championships. He placed third in the short program, winning a bronze small medal, despite a minor error on his triple Axel.
2019–2020 season
Grassl began his season on the Junior Grand Prix in Poland, where he won the bronze medal. His second event was the Italian JGP event, held in his home rink, where he won the gold medal, qualifying to the Junior Grand Prix Final. Moving to the senior level Challenger series, Grassl won both the Asian Open and the Ice Star. At the JGP Final in Torino, Grassl was the lone Italian skater to qualify at either senior or junior level. Two days before the competition was to begin, his right skate broke, which he attempted to remedy with tape. This proved inadequate in skating the short program, where he placed fifth after missing the second part of his jump combination. Grassl then replaced his skates with only a day's preparation, and struggled in the free skate, finishing last among the competitors. Shortly afterward, he won his second consecutive Italian national title. 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 medalistPetr Gumennik.
Records and achievements
At the age of 15, Grassl became the youngest person to land a quad Lutz jump in international competition. His record was broken by Stephen Gogolev in 2018.
The first European skater to have completed a quad loop in international competition. He landed the jump in his free skate at the 2018 Golden Bear of Zagreb.
