Champions Juve scored twice in as many minutes late in the second half through Jose Martin Caceres and Paul Pogba to win a hotly-contested match in Turin that saw Antonio Conte's side stretch their unbeaten league run to 46 games.
Caceres netted a powerful header after 80 minutes and Pogba wrapped up the victory over second-placed Napoli with a stunning volley two minutes later to score his first goal for the club.
"It was a difficult match for us. We deserved to win," Caceres told Sky Sport Italia. "It's fundamental for us that we can look at the bench and ask for them to change a match."
"This doesn't change anything however, this is only three points. There are still 30 matches to go."
Sebastian Giovinco was a constant thorn in Napoli's side and should have given Juventus the lead 10 minutes into the second period but, after working some space in a tightly packed area, he blasted his shot wide with the goal at his mercy.
Juve's Alessandro Matri also wasted a good chance after 65 minutes when he failed to control a beautiful long pass from Andrea Pirlo eight metres out with only De Sanctis to beat.
However, Conte's determined team were not to be denied and with 10 minutes to go Caceres headed firmly into the net from Pirlo's corner two minutes after coming on for Kwadwo Asamoah.
Lazio took the lead in the 24th when Hernanes' smart dribbling ended with his shot being deflected by Daniele Bonera over the stranded Marco Amelia for his fifth goal this season.
Milan almost equalised straight away when Stephan El Shaarawy fired the ball between the legs of onrushing Lazio keeper Albano Bizzarri, only to see Andre Dias's last-ditch slide block his goal-bound shot.
Miroslav Klose piled on the misery for Milan coach Massimiliano Allegri four minutes into the second half, finding a yawning gap in their defence and beautifully volleying Candreva's pinpoint long pass home.
