Champions Juventus thumped down-hearted Udinese 4-1 away after the hosts, still coming to terms with their failure to qualify for the Champions League, had goalkeeper Zeljko Brkic controversially sent off after 13 minutes.
Napoli beat Fiorentina 2-1 on a pitch which resembled a ploughed field and Cagliari drew 1-1 with Atalanta behind closed doors because their new venue for this season was not considered ready by local authorities.
The Is Arenas stadium outside Cagliari was only confirmed as the venue on Friday night after the home players protested at plans to stage the match in Trieste, around 1,000 kilometres away on the mainland.
Cagliari, who hosted several matches in Trieste last season, were not allowed any spectators with one half of the arena still clearly under reconstruction.
Roma went ahead at the San Siro when Alessandro Florenzi, unmarked in the penalty area, met Francesco Totti's cross with a looping header in the 15th minute.
Cassano, left out of the Italy squad on Sunday, levelled with a deflected shot in first-half stoppage time for Inter, who won their opening match 3-0 at Pescara.
It was the volatile striker's first goal since his move from AC Milan in a swap with Giampaolo Pazzini, who scored a hat-trick for his new side in their 3-1 win at Bologna on Saturday.
Roma, who have four points, regained the lead with a stunning goal as Osvaldo was sent clear of the defence in the 67th minute and deftly flicked the ball over goalkeeper Luca Castellazzi. Marquinho added a third with nine minutes left.
Osvaldo, however, blotted his copybook with a second yellow card for a needless handball at the end.
Udinese goalkeeper Brkic was dismissed after clattering into Sebastian Giovinco as the former Parma striker rang on to a long Andrea Pirlo pass.
Arturo Vidal fired the penalty past new goalkeeper Daniele Padelli to set Juventus on the way to an easy win and Mirko Vucinic curled in the second before halftime.
Giovinco scored two in the second half to bury Udinese, who were beaten in their Champions League play-off by Braga to fall at the same stage for the second season running.
"Juventus can win in Udine but deciding the match after 15 minutes does nobody any favours, including Juve," said Udinese owner Giampaolo Pozzo, whose team have lost their first two games.
"The red card was completely unnecessary. A decision like that requires a little common sense and balance."
Juventus, unbeaten last season, have six points from two games along with Napoli and Lazio.
Second-half goals from Marek Hamsik and Blerim Dzemaili gave Napoli a 2-1 win over Fiorentina on a San Paulo pitch which was heavily sanded, full of ruts and divots and appeared to be nearly unplayable.
Stevan Jovetic replied for the visitors.
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