First-half goals from Ezequiel Lavezzi and Christian Maggio sunk Udinese who went into the midweek games top but are now in second place. They are one point behind undefeated leaders Juventus, 2-1 winners at home to Fiorentina on Tuesday.
Third-placed Lazio, on the same total of 15 points as Udinese, missed the chance to climb to the summit when they were held to a 1-1 home draw by Catania after Miroslav Klose had put the hosts ahead in the 17th minute.
Midfielder Antonio Nocerino scored a hat-trick in AC Milan's 4-1 win over Parma, the second time in four days the defending champions scored four goals, while struggling Inter Milan drew 1-1 at Atalanta.
Milan are fifth, equal on 14 points with Napoli.
Napoli have been spectacularly erratic this season, enjoying convincing wins against Milan and Inter and losing at home to lowly Parma.
Udinese, missing top-scorer Antonio Di Natale through injury, were unlucky to face Walter Mazzarri's team on one of their good days and were on the back foot from the start.
Marek Hamsik had a goal disallowed in the 16th minute but Napoli only had to wait another four minutes to take the lead.
Edinson Cavani flicked the ball over Lazio's back line and found Lavezzi, who brilliantly hooked the ball into the net on the turn.
Napoli created further chances although Udinese, who went top on Sunday, were unlucky not to equalise when Morgan de Sanctis made outstanding saves to turn away two goalbound shots.
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The home side wrapped the game up before half-time when Blerim Dzemaili's cross was headed in by Maggio at the far post.
Lazio appeared set to go top with a fourth straight win when Klose struck early but they then gifted Catania an equaliser, a defensive blunder allowing Argentine Gonzalo Bergessio to level in the 63rd minute.
Milan, who had to come from three goals behind to beat Lecce 4-3 on Sunday, made life much easier for themselves against Parma.
Nocerino turned in Zlatan Ibrahimovic's cross on the half-hour and fired a second goal from outside the area two minutes later to put them in command.
Kevin-Prince Boateng grabbed a hat-trick against Lecce and Nocerino emulated him with an injury-time header.
Ibrahimovic was also on target while Sebastian Giovinco replied for Parma.
Dutchman Wesley Sneijder gave Inter the lead at promoted Atalanta after 32 minutes but Argentine German Denis equalised a minute before halftime.
Denis then missed a penalty in the closing stages.
Inter are 16th on eight points, one point behind Atalanta who had six points deducted over their involvement in a match-fixing scandal in Serie B last season.
AS Roma are eighth with 11 points after they lost 2-1 at Genoa.
