[4] He made his Serie A debut on 7 September 1996 with AS Roma in a 3–1 win over Piacenza Calcio, and would be an instrumental figure in the side's 2001 conquest of the scudetto, with manager Fabio Capello even labelling him as the team's most important player.

[8] He finally returned to play on 30 October 2005, coming on as a second-half substitute for Olivier Dacourt during a league match against Ascoli Calcio 1898 and being hailed with a long standing ovation by the Roma supporters.

After ten years with the club, in July 2006 he joined Levante UD in Spain,[10] spending two seasons with the La Liga strugglers, eventually ending in relegation in 2007–08.

[11] On 9 January 2009, his contract was terminated by mutual consent and, after advanced talks with Chinese Super League's Tianjin TEDA, he signed for the club early in the following month, citing an interest in a third experience abroad as the main reason for it.

[12] After one season, 35-year-old Tommasi left Teda and decided to return to Italy, joining amateurs Sant'Anna d'Alfaedo (Seconda Categoria), where he played alongside his two brothers.

[15] Tommasi played for the Italy under-21 team that won the 1996 UEFA European Championship, also being picked for that year's Summer Olympic Games in Atlanta where he made three appearances.

After featuring prominently during the Azzurri's 2002 FIFA World Cup qualifying campaign, Tommasi was picked by manager Giovanni Trapattoni for the squad that would take part at the finals in Japan and South Korea.