Napoli ceded control of the Serie A title race to reigning champions Juventus after failing to beat Inter Milan at San Siro.
Maurizio Sarri's Azzurri led the table by a point on Sunday morning but were deposed when Juve defeated Udinese 2-0 in the afternoon.
Three points from their evening trip to Milan would have restored their status as front-runners for the Scudetto, which they have not won since 1990, but ambitious Inter held firm to force a goalless draw.
Napoli could even find themselves four points adrift of Juve should the Bianconeri make the most of their game in hand against Atalanta, which takes place in Turin on Wednesday night.
The first half at San Siro was largely a tactical battle but, as half-time drew nearer, the game was wide open.
Napoli started enthusiastically, with Allan dribbling his way into the Nerazzurri box before savvy defending denied him the chance to shoot.
Inter found themselves packed into their own half until the half-hour mark, with Milan Skriniar's block on Lorenzo Insigne finally allowing them to break forward.
The ball was pinging back and forth until Inter schemer Antonio Candreva took control of possession and fizzed a drive inches wide of Jose Reina's post.
Shortly after the restart Slovakian centre-back Skriniar headed against the Napoli bar from a Joao Cancelo free kick and the visitors responded immediately as Insigne curled the ball just off target.
The Italy striker continued to be a menace and in the 67th minute he was unfortunate when trying and failing to net with a volleyed swipe at Dries Mertens' clever assist.
A mistake by Napoli allowed Mauro Icardi to snaffle the ball and take aim but Reina stood firm to collect, and soon the Azzurri were threatening again as another Insigne attempt bounced off Skriniar.
Inter's last opportunity to score arrived in the 86th minute when Cancelo swung a free-kick into a packed Napoli box. However, Reina punched clear with confidence and the points were shared.