Napoli struggled to get the ball out of their half but almost drew level against the run of play in the 13th minute.
Vladimir Petkovic's side remained the better team and came agonisingly close to doubling their lead just after the half-hour mark, with Floccari's right-footed drive coming back off the post.
Napoli hit the woodwork at the other end moments later. Edinson Cavani leaped high in the box to meet Goran Pandev's corner but his header rebounded off the crossbar.
Lazio claimed the first attack of any note in the second half, coming close to landing another blow in the 56th minute through Alvaro Gonzalez, who shot wide to the left from outside the box.
Gokhan Inler and Cavani both squandered opportunities for Napoli and Pandev came close to drawing them level in the 82nd minute when his header from Campagnaro's cross produced a good save out of Marchetti.
Walter Mazzarri's side left it late to salvage a point. Omar El Kaddouri's 87th-miute cross fell invitingly for Campagnaro inside the box and he found the top corner to ensure a share of the spoils.