Grassotto entered karting competitions in 1994 at the age of 14. He achieved early results that demonstrated his potential at the national level, including second place in the Cadets class of the Rainbow Trophy in 1994 and a third-place finish in the Torneo delle Industrie 100 Junior class the same year. His most notable karting result came in 1997 when he finished second in the ICA class of the Trofeo Andrea Margutti, one of the most prestigious karting events in Europe. In 1998 he made an appearance at the FIA World Karting Championship Formula A without scoring.
Grassotto moved into single-seaters in 1998, finishing runner-up in the Formula Renault Campus Italy series in his debut season. His breakthrough in international competition came in 2000 when he raced in the Formula Renault 2.0 Eurocup for JD Motorsport, driving a Tatuus FR2000. He finished third overall in the Eurocup with 116 points from nine races, scoring one win at Circuit Park Zandvoort, three further podiums, and two pole positions, competing against a field that included the eventual champion Felipe Massa. He also made a partial campaign in the Formula Renault 2.0 Italy championship that year, finishing 12th with one podium. Additionally, Grassotto entered the 2000 Masters of Formula 3 at Zandvoort for Target Racing in a Dallara F300, finishing tenth.
In 1999, Grassotto raced with Lucidi Motors in the Italian Formula 3 Federale National Trophy, clinching the championship title two races before the season's end. He stepped up to the German Formula Three Championship in 2001 with the Italian team Cram Competition, driving a Dallara F301 powered by an Opel engine. Over 13 races he accumulated 43 points and finished 11th in the drivers' standings, with two podiums. His best results came at the Sachsenring, where he finished third in Race 2, and at Hockenheim, where he finished third in Race 1 — a round held as a support event to the German Grand Prix.
Grassotto spent the 2002 and 2003 seasons in the Euro Formula 3000 series, competing with Draco Junior Team and Traini Corse respectively in the Lola T99/50-Zytek. In 2002 he scored two podiums and finished seventh in the championship with 12 points. His best season in open-wheel racing came in 2003 when he claimed four podiums and finished fourth in the standings with 23 points, establishing himself as a frontrunner in the regional series.
He stepped up to the FIA International Formula 3000 Championship in 2004 with Spero I.E./AEZ IE Engineering in a Lola B2/50-Zytek, making two race starts. He scored his sole point of that campaign with an eighth-place finish at Spa-Francorchamps before retiring from the Monza round. He also contested the non-championship Formula 3000 Grand Prix of Portoroz the same year, finishing seventh. A single appearance followed in the 2005 Italian Formula 3000 with BCN Competition, where he did not score. Across his Formula 3000 career he started 22 races, totalling six podiums and 36 points.
After a gap from 2006 to 2007, Grassotto switched to GT racing in 2008 with the Italian squad Mik Corse, driving a Lamborghini Gallardo GT3 shared with co-driver Giacomo Piccini. He won the Italian GT Championship GT3 class title that season with 132 points, beating out rivals including Stephen Earle of Kessel Racing. He also entered the Bomboogie GT Challenge that year, finishing fourth. Grassotto returned with Mik Corse in 2009, primarily sharing the Gallardo with Adriano Baso, and finished fifth in the GT3 standings with 112 points.
Following his final competitive season in 2009, Grassotto retired from professional racing and went on to operate the Friuli Venezia Circuit, a dedicated karting complex in Italy.