Inter gave new coach Leonardo a flying start to his San Siro career with a 3-1 win at home to Napoli as neighbours AC Milan moved five points clear at the top with a 1-0 win at Cagliari.
Former Milan boss Leonardo, who replaced Rafael Benitez, saw Thiago Motta grab a brace while Esteban Cambiasso added the other to sink Napoli, who replied through Michele Pazienza.
Inter are still 13 points behind Milan in seventh but they do have two games in hand.
Leonardo revealed that the moment almost passed him by while he refused to make predictions on the season.
"It was very fast and wonderful, an intriguing situation," he said of his brief spell at the club helm.
"I want to maintain the satisfaction of a winning team. (But) we need to think about one step at a time, game after game."
Napoli dropped to third, six points off the leaders and just one behind Lazio who drew 0-0 at Genoa.
Inter enjoyed the best possible start at the San Siro as brilliant one touch football between Goran Pandev and Dejan Stankovic sent Motta through to hit a first time volley beyond Morgan De Sanctis on three minutes.
Napoli had the ball in the net moments later but Edinson Cavani's close-range header was disallowed as Andrea Dossena had strayed offside before crossing.
However they were level on 25 minutes as Pazienza headed home an Ezequiel Lavezzi corner.
Cavani then teed up Christian Maggio who should have done better than drag his shot wide of the far post.
On 35 minutes Diego Milito should have given Inter the lead but somehow spooned over from six yards.
Yet two minutes later they were in front as Cambiasso arrived late and at pace in the box to head home Maicon's cross.
Napoli's Marek Hamsik stabbed wide from an early second half chance before it was three as Motta sent a brilliant header into the top corner on 55 minutes.
Inter captain Javier Zanetti had the best chance thereafter but pulled his effort across goal.
Sierra Leone international Rodney Strasser pounced five minutes from time to score the only goal of the game in Sardinia as AC Milan beat Cagliari 1-0.
Controversial signing Antonio Cassano, on his debut as a substitute, slipped in the 20-year-old to beat Michael Agazzi in the Cagliari goal.
It was Strasser's first ever goal for Milan.
"Things look good after winning a tough game like today," said Milan coach Massimiliano Allegri.
"Most of all with a goal from a young lad who's come through our youth section, I think we have reason to be pleased.
"And I'm pleased with the table and for having started the year a little better than we finished the last one (with a 1-0 home defeat to Roma)."
Roma leapfrogged Juventus into fourth as they came back from 2-1 down to beat Catania 4-2 in Rome thanks to doubles by Marco Borriello and Mirko Vucinic.
On-loan forward Sebastian Giovinco sent out a message to his regular employers Juventus as he scored a brace for Parma.
Having struggled for the last couple of years to establish himself at Juve, Giovinco went on loan to Parma this season in a bid to find first team football.
But he came back to haunt his permanent employers with a well taken double that badly hits Juve's title chances as they dropped to eight points behind Milan.
Following Giovinco's brace Juve centre-back Nicola Legrottaglie pulled a goal back but Hernan Crespo's penalty sealed the three points before substitute Raffaele Palladino's injury-time finish added insult to injury.
Juve were eventually overrun largely due to playing most of the game with 10 men following Brazil midfielder Felipe Melo's sending off for violent conduct after only 17 minutes.
At the bottom of the table basement sitters Bari boosted their slim hopes of survival by winnning 1-0 at second bottom Lecce while Cesena climbed out of the drop zone with a 2-1 win at Brescia, who replaced them.