Nene gave the hosts the lead, but a Gonzalo Higuain penalty brought Rafael Benitez's side level.
Napoli had a goal disallowed as they searched in vain for a winner, reducing the arrears on Roma to two points but allowing Fiorentina the opportunity to move to within three points with victory at Sassuolo on Sunday.
Nene was heavily involved in the opening exchanges, producing a clinical early finish which rather than putting Cagliari in front, earned him a yellow card since play had long since been stopped for offside.
There was no doubting his opener in the ninth minute, though, as he tucked the ball under Pepe Reina after Christian Maggio slipped and allowed Davide Astori to pull the ball back from the goal line.
Nene's game ended soon after as he had to leave the action due to injury, while Astori then ruined his good work in creating the goal by gifting one to Napoli by fouling Goran Pandev inside the penalty area.
Higuain stepped up to beat Vlada Avramov and bring Napoli level.
Reina did not re-emerge for the second half, Rafael taking his place and being largely a spectator in a second half dominated by Benitez's men.
All that they lacked was a goal to reward their superiority, although it was not for the lack of trying.
They did have the ball in the back of the net in the 73rd minute when Jose Callejon's volley passed through a pack of players on its way in, but the linesman's flag was raised to signal an offside against Higuain.
Napoli's last chance was missed by Dries Mertens, whose angled shot curled wide of the far post.