Hellas Verona won 3-1 at Udinese helped by two goals from Luca Toni, who netted the first after a blunder from home goalkeeper Zeljko Brkic.
Brkic allowed an apparently harmless Toni shot to slip between his legs in the eighth minute.
Parma defender Alessandro Lucarelli provided the day’s highlight when he scored with a cheeky backheel following a corner as Roberto Donadoni’s team came from behind to beat Torino 3-1. Striker Amauri scored his first goal of the season to complete the scoreline.
Bottom club Catania beat Bologna 2-0 in a relegation scrap while improving Genoa overcame lowly Sassuolo by the same score. Lazio were at home to Inter in the late kickoff (1730 GMT).
Roma lost 3-0 to leaders Juventus in the top-of-the-table clash on Sunday.
Napoli’s win lifted them to 39 points from 18 games, two behind Roma, with titleholders Juventus on 49, a massive eight clear at the top.
Mertens provided the breakthrough for Napoli when he volleyed in Gonzalo Higuain’s cross in the 53rd minute, just as Sampdoria were threatening to take control, and the Belgian curled in a 25-metre free kick nine minutes later.
Napoli coach Rafael Benitez said that his team nearly blew it again after wasting too many chances.
“We had many scoring opportunities and could have made the difference in the first half. The game was more open in the second and they caused us problems, but we got the goals,” he told reporters
“I would like to see us stronger in defence and quicker at finishing games off but it’s reassuring to see such attacking football.”
Monday’s games were played amid concern about Serie A leading goalscorer Giuseppe Rossi after he suffered a third injury to his troubled right knee in Fiorentina’s 1-0 win over Livorno on Sunday.
Fiorentina said on Monday that Rossi had sustained a second degree sprain to the medial collateral ligament. Although worrying, there was relief that he avoided damage to the anterior cruciate ligament, which has been operated on twice and cost him two years of his career.
Milan, whose coach Massimiliano Allegri confirmed over the Christmas break that he would step aside at the end of the season, climbed to a modest 10th on 22 points.
Former World Player of the Year Kaka, who returned for a second stint at Milan at the start of the season, put them in front 10 minutes before the break with his 100th goal for the club in all competitions.
The Brazilian swept home Urby Emanuelson’s cross from a tight angle after Milan broke following an Atalanta corner.
Yohan Benalouane had a goal disallowed for Atalanta and German Denis was denied by Christian Abbiati as Milan showed signs of faltering before Kaka struck again, firing home a loose ball after Robinho scuffed a Mario Balotelli cross in the 65th minute.
Two minutes later, Cristante celebrated his first Serie A start by scoring a pinpoint 25-metre shot which went in off the foot of the post.
At the bottom, Catania have 13 points, one behind Livorno and Sassuolo and only two adrift of 17th placed Bologna.
(Reporting by Brian Homewood, editing by Tony Jimenez and Pritha Sarkar)