Flying AS Roma beat Palermo 4-1 on Saturday to keep up the pressure on Inter ahead of the meeting between the sides in Rome in late March.
AC Milan, nine points behind Inter in third with a game in hand, edged out Udinese 3-2 on Friday to leave the champions with work to do having only drawn at Parma on Wednesday.
Fourth-placed Napoli's fanatical supporters showed their love for the club on Valentine's Day by packing the San Paolo and creating an intimidating atmosphere for Inter.
Marek Hamsik, a transfer target for the leaders, rattled the bar early on amid a host of early chances for the home side while Inter had a decent penalty shout for handball turned down.
Jose Mourinho's side then struck the woodwork with a Sulley Muntari firecracker before Goran Pandev missed a good chance in the second half and Napoli's Fabio Quagliarella thumped a drive against the post.
Earlier, Alessandro Del Piero converted a controversial penalty to give Juventus a 3-2 home win over Genoa and lift the Turin club to sixth after only their second victory in nine Serie A matches.
LAZIO CLIMB
Juve caretaker coach Alberto Zaccheroni, who drew his first two games in charge, again opted for three in defence but Genoa sliced through the back line 16 minutes in and Marco Rossi's scuffed effort bobbled into the corner.
Amauri ended his mini goal drought by equalising just before the break and strike partner Del Piero finished off a fine move to put the hosts ahead midway through the second half.
Gianluigi Buffon then spilled a cross to let Rossi tap in his second before Del Piero won and scored a penalty 12 minutes from time amid protests that Sokratis Papastathopoulos had not tripped the forward and any contact was outside the area.
"It means so much because every match at the moment is crucial for us considering the situation we've put ourselves in," said 35-year-old Del Piero, who was making a record 445th league appearance for Juve.
Guglielmo Stendardo and Mauro Zarate netted in the second half to hand Lazio a 2-0 win at Parma in Edy Reja's first game in charge of the Rome side since replacing Davide Ballardini.
Lazio, who climbed out of the bottom three, included Argentine midfielder Cristian Ledesma for the first time this season having been banned from the squad for asking for a move.
Cagliari beat mid-table rivals Bari 3-1 while bottom side Siena boosted their outside hopes of survival by winning 1-0 at Chievo.
Bologna also prevailed 1-0 at third bottom Livorno and Catania drew 0-0 with fellow strugglers Atalanta.
