While Milan marched on, champions and city rivals Inter Milan beat Lazio 2-1 to move up to second place, ahead of Napoli who lost by the same score at Palermo, while Bari were relegated.
AS Roma beat Chievo Verona 1-0 to consolidate sixth place and close in on Udinese, three points ahead, and fourth-placed Lazio.
Cassano should have given Milan the lead four minutes before the break when he somehow nodded over from eight metres after being picked out by Kevin-Prince Boateng's cross.
"We should have finished them off in the first half," Milan coach Massimiliano Allegri told Sky Italia. "There's nothing to celebrate yet, but we'll have a nice Easter."
Wesley Sneijder then equalised with a free-kick five minutes before half-time and Samuel Eto'o scored the winner, his 20th goal of the season, on 53 minutes after rounding goalkeeper Fernando Muslera.
"I'm an optimist and I still think it (the title race) is open," Inter coach Leonardo told Italy's Sky channel before the Milan result.
"We had one bad week in four months which can happen. It was an amazing win and showed just how committed this squad is."
Home coach Delio Rossi was sent from the dugout as tempers flared but Cesare Bovo's penalty on the stroke of half-time after Giulio Migliaccio had been felled in the box turned out to be the winner for Palermo, who stayed eighth.
"The boys were a bit tense," said Napoli coach Walter Mazzarri. "We're not used to being so high up the table and some players are able to handle the pressure better than others."
Fifth-placed Udinese failed to take advantage of Lazio's loss, losing 2-0 at home to Parma with an Amauri double g
