Gonzalo Higuain equalised for Rafael Benitez's men with a 19th-minute penalty, his ninth league goal of the season, after Nene had put 11th-placed Cagliari in front early on.
Napoli were unlucky in the 72nd minute when a stunning volley from Jose Callejon was disallowed for the tightest of offside decisions.
Benitez's team have 36 points from 17 games, two behind unbeaten AS Roma who host Catania on Sunday and seven adrift of Juve.
Goals from Nicolas Lopez and Thomas Heurtaux saw 10th-placed Udinese win for the first time in four matches, 2-1 at third from bottom Livorno.
Francesco Guidolin's side moved a point clear of AC Milan who play city rivals Inter on Sunday.