Milan's thrilling 3-2 derby victory over Internazionale on Saturday night strengthened their grip on the title but Roma moved back to within five points by crushing Siena 6-0 at home.
Cassano was the hero for Fabio Capello's troops as he netted a stunning hat-trick in a match which both teams finished with ten men.
Italian international Cassano opened the scoring on 20 minutes and netted his second just five minutes later.
Alessandro Mancini further extended the lead on the half hour before Roma's Jonathan Zebina was sent off early in the second half.
Francesco Totti missed a penalty on 56 minutes before Cassano completed his hat-trick in the 71st minute, just five minutes before Siena's De Souza Juarez was dismissed for a second bookable offence.
Marco Delvecchio rubbed further salt in to Siena's wounds with a fifth goal on 82 minutes and Totti made amends for his earlier miss from the spot by rounding off the rout three minutes from time.
Juventus kept alive their hopes of retaining the Scudetto after Mark Iuliano's 57th minute goal snared the points at Bologna.
Perugia finally won their first Serie A match of the season - at the 22nd attempt - with a last-gasp 2-1 success at Reggina.
Ze Maria put the visitors ahead on 19 minutes but Francesco Cozza's 54th minute penalty levelled the scores. Veteran striker Dario Hubner ended Perugia's sorry run by grabbing the club's first winning goal of the campaign in the final minute.
Lazio failed to boost their UEFA Champions League aspirations after a 0-0 draw with Chievo in Verona. Marco Malago was sent off for the hosts on 58 minutes and Chievo were indebted to Claudio Lopez missing a penalty for a point.
Ancona remain firmly adrift at the foot of the table after a 3-0 defeat at Udinese ensured they are now the only team in Serie A without a victory this season.
Dino Fava's 58th minute goal broke the deadlock and goals inside the final ten minutes from Marek Jankulovski and David Pizarro sealed the success.
Lecce moved out of the relegation zone after hanging on for a 2-1 success at Brescia.
In-form Javier Chevanton opened the scoring for Lecce on 12 minutes and the visitors appeared to sew up the match when Marco Cassetti netted on 87 minutes. But Lecce had to weather a late rally from the hosts after Roberto Baggio scored in the final minute.
Empoli failed to seriously enhance their survival prospects despite battling back for a point at Modena.
Nicola Amoruso put Modena ahead on 38 minutes and Empoli carved out a point through Tommaso Rocchi five minutes after half time.
Click below for an individual match summary:
Bologna 0-1 Juventus
Brescia 1-2 Lecce
Chievo 0-0 Lazio
Modena 1-1 Empoli
Reggina 1-2 Perugia
Roma 6-0 Siena
Udinese 3-0 Ancona
