Born in Florence, Flachi made his professional debuts with local giants ACF Fiorentina, appearing in ten Serie B games and scoring twice as the 1993–94 season ended in promotion.
Flachi would appear rarely for the Viola in the top flight, however, facing stiff competition from the likes of Gabriel Batistuta and Luís Oliveira - only three league games in each of his three last seasons.
In the following campaign, he continued to display solid performances (again scoring in double digits), this time accompanied by Emiliano Bonazzoli up front; however, the team failed to qualify from the UEFA Cup group stage, and finished 14th in the league (12th after the match-fixing scandal).
He played 14 games, mostly as a substitute, until he was suspended again in January 2010 following a positive for cocaine in a test held on 19 December of the previous year; in June, he was handed a 12-year ban, effectively ending his career as a player.
[6][7] In 2021, Flachi announced he had been training with the Italy fifth-tier team Signa 1914, and was planning to make a comeback as a player when his ban expired in January of 2022.
[9] He successively moved back to Genoa and signed for Promozione amateurs Praese during the summer of 2022,[10] with whom he finally scored his first league goal since his comeback in October at the age of 47.