The Italian edged out compatriots Massimo Graziato of Lampre-ISD and Filippo Baggio of Utensilnord-Named to take the initial green jersey as the six-stage tour got underway with the first part of a double-header..
The stage 1a win is 30-year-old Rossi's seventh victory of 2012.
A deflated runner-up Graziato said: "The final stretch proved to be a very complicated, winding road, made treacherous by heavy rain that soaked the asphalt.
"We studied it for the first time when we passed the finish line, then when we approached the final sprint I took the wheel of Petacchi.
"I went right to the last corner 150 meters from the finish, unfortunately Rossi was very fast."
Later in the day, Colnago-CSF finished fastest in the 18km stage 1b team time trial from San Giovanni in Persiceto Crevalcore.
Directed by Bruno and Roberto Reverberi, Colnago's time of 20 minutes and 23 seconds was four faster than Androni-Venezuela in second and seven ahead of third-placed Liquigas-Cannondale.
The 1b result meant that Sonny Colbrelli became the new race leader.
Stage two takes place on Tuesday and involves a mostly-flat 220km ride from Poggio Renatico to San Vendemiano.