Coached by Magri,[5] 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.

In November, he stepped onto two more senior international podiums, taking gold at the Ice Challenge in Austria (27.33 points ahead of silver medalist Javier Raya from Spain) and then bronze at the Merano Cup in Italy.

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.

At the JGP Final in Torino, Grassl was the lone Italian skater to qualify at either the senior or junior level.

[12] Staging a comeback in the free skate, he was second in that segment with a new personal best score and rose to fourth place overall, under two points short of the silver medal.

[15] Grassl was scheduled to make his senior World Championship debut in Montreal, but these were cancelled as a result of the coronavirus pandemic.

[16] Grassl won the 2020 CS Budapest Trophy and was assigned to make his Grand Prix debut at the 2020 Internationaux de France, but this event was also cancelled as a result of the pandemic.

[17] Grassl won his third consecutive Italian national title in December, after which he tested positive for COVID-19, as a result of which he was off the ice for a month and a half due to quarantine and subsequent concerns about the condition of his heart.

[19] Grassl and Matteo Rizzo's placements qualified two berths for Italian men at the 2022 Winter Olympics in Beijing, China.

[21] Rizzo later withdrew due to a positive COVID test, as a result of which Grassl was the lone Italian man in the competition.

[26] His second Grand Prix assignment was initially the 2021 Cup of China, but following that event's cancellation, he was reassigned to a special home 2021 Gran Premio d'Italia in Turin.

He placed second in the free skate, despite both of his triple Lutzes being called for incorrect edges, setting new personal bests in that segment and in total score and taking the silver medal.

[34] In the free skate, Grassl made only minor errors in a three-quad program, managing a new personal best that saw him fourth in that segment, rising to seventh overall.

[30] He fared better at his second event, winning gold at the 2022 MK John Wilson Trophy, which result in turn qualified him for the Grand Prix Final, becoming the first ever Italian male figure skater to do so.

[41] A week later, he competed at the 2022 CS Warsaw Cup and won the silver medal, finishing less than half a point behind event champion Kévin Aymoz.

[30] Shortly before the Grand Prix Final, Grassl announced that he had moved back to Egna to train due to homesickness.