First-half goals from Mirko Vucinic and Alessandro Matri in Turin took Juventus to 55 points, with Napoli six points ahead of third-placed Lazio after Hugo Campagnaro struck an 87th-minute equaliser to earn a point at the Olympic Stadium.
Vucinic opened the scoring for a combative Juve side after 20 minutes, capitalising on Facundo Roncaglia's poor defensive header by smashing the ball home from just outside the area.
"It wasn't an easy match, Fiorentina play great football but we deserved the win. We were brilliant, extremely aggressive," said Juventus director and former player Pavel Nedved.
Sergio Floccari scored Lazio's opener 11 minutes into a thrilling match with Napoli when he controlled Abdoulay Konko's cross on the penalty spot and hit a fierce low drive past Morgan De Sanctis.
The home side deserved their lead against a below-par Napoli, after a first half of attacking football that Walter Mazzari's team could not handle.
"We played some brilliant football and created loads of chances, it was a spectacular match," said Lazio manager Petkovic. "We had a number of chances to close the match out but we couldn't do it and we were punished for it."
Despite Napoli's poor showing, Edinson Cavani almost grabbed an equaliser in the 38th minute when he met Juan Zuniga's cross with a thumping header that evaded Lazio keeper Federico Marchetti but hit the crossbar.
Both sides had chances to win in a frantic final few minutes as Napoli's Goran Pandev saw Marchetti save his low drive in the 90th, while in second minute of injury time Floccari again found himself in space only to crash his header against the bar.
