Strikes from the Italy international in the 11th and 17th minute saw the Viola race into a two-goal lead and, though Manolo Gabbadiani pulled one back late on, the hosts held on to gain all three points.
Rossi had been out of action for over a year with a knee injury but is making his mark in Serie A this season after signing for Vincenzo Montella's side in January last season from Villarreal.
And after being named in Cesare Prandelli's Azzurri squad to face Germany and Nigeria, he stressed that his form and fitness are continuing to improve.
"I feel good, today I played 90 minutes," the 26-year-old told Mediaset Premium. "I can give more, but in every game I always feel better.
"Earlier this season we gave away some points, both against Parma and Napoli [but not today].
"I hope to be a fundamental part of the Italy team that will participate in the World Cup. Every game I try to do my best to get results. This [Italy call-up] is one more incentive which gives me satisfaction."
Fiorentina sit fifth in Serie A - eight points behind league leaders Roma - and trail Inter by one point.