Earlier, visiting keeper Nicolas Navarro did well to keep out a David Trezeguet shot from close range and an Alessandro Del Piero free kick.
Napoli's eighth consecutive away defeat will increase the pressure on coach Edy Reja, whose side have dropped to 11th after picking up two points in eight games.
Inter can regain their nine-point advantage over second-placed Juve by beating AS Roma at the San Siro on Sunday.
"We played an excellent first half. After that Napoli came out, they lost their initial nervousness and poured into our half," Juventus coach Claudio Ranieri told Sky television.
"The goal at the end of the first half was providential. We built our victory on that."
Elsewhere, Mauro Zarate struck twice as Lazio beat struggling Bologna 2-0 to move up to seventh.
The Argentine forward ended his goalless spell, which stretched back to December, by curling in a free kick on 36 minutes and chipping his second with nine minutes to go after a one-two with Tommaso Rocchi.
Lazio's first league win at home in two months made it a bitter return to the Stadio Olimpico for Bologna boss Sinisa Mihajlovic, who played for the Rome side between 1998 and 2004.
Midfielder Massimo Mutarelli came on in the second half to make his debut for Bologna against the club he took to a tribunal to free himself from his contract.
