Inter Milan's Guarin celebrates with his teammate Cassano after scoring against Napoli during their Serie A soccer match at San Siro stadium in Milan (Photo: Reuters)
A pair of goals from south Americans Fredy Guarin and Diego Milito settled Inter's nerves before half-time and despite Uruguayan striker Edinson Cavani reducing the arrears just before the hour the visitors failed to pull level in a tighter second half.
With the players' still warming up in freezing temperatures at the San Siro, the roof was raised when Guarin ghosted in at the back in the eighth minute to volley Antonio Cassano's curling corner past a helpless Morgan De Sanctis.
Napoli looked dangerous going forward but spurned the few half-chances they created.
Slovakian playmaker Marek Hamsik sent an angled shot wide from 10 yards from Cavani's lay-off and Cavani was ruled offside after a missed header from close range after Lorenzo Insigne's perfect delivery at the back post.
Cassano, meanwhile, was making himself a nuisance for the Napoli defence and twice got into the area after one-twos with Milito, only to be left needing treatment after being sandwiched between Cannavaro and Gamberini.
When Inter scored their second, it came from nothing but put the Nerazzurri well on their way to securing the three points.
The lively Guarin pounced on a poor Napoli clearance, strode forward to feed Milito who did well to hold off Alessandro Gamberini and sidefoot the ball into the net despite a desperate parry by De Sanctis.
Inter emerged in positive fashion after the interval, although Guarin drove his long-range effort from Alvaro Pereira's pass well wide and when the Uruguayan fed Cassano the Italian hit his close range shot straight at De Sanctis.
Napoli, however, pressed forward and when the ball fell to Gohkan Inler on the edge of the area the Swiss midfielder's well-struck volley brought a diving save from Samir Handanovic.
Napoli had the ball in the net moments later with Cavani pouncing at the back post after Samir Handanovic had performed heroics to somehow keep out desperate efforts. The goal galvanised Napoli but their claims for a penalty after Christian Maggio had fallen under pressure from a defender were waved away.
Andrea Stramaccioni bolstered his Latin contingent when he replaced Cassano with Rodrigo Palacio on the hour mark, and the Argentine had an almost immediate effect when he nodded down for Guarin only for the latter's shot across goal to find no takers.
Napoli had stalled and after an Inter counter the hosts held court in the visitors' area but when Guarin found time and space after deftly sidestepping defender Valon Behrami his through ball went astray.
Milito made way for Brazilian midfielder Coutinho but it was wingback Yuto Nagatomo who next threatened De Sanctis, the keeper being forced to block as the Japanese chased down Guarin's chip with seven minutes to play.
Inter lived dangerously at the other end when Cavani powered down the left and sent in a cross which a backtracking Pereira somehow spun out for a corner.
Cavani then won a freekick and when Insigne sent in an inviting curling ball for Maggio at the back post his headed effort went straight to Handanovic.
Inter, however, had the last word although after Palacio had weaved his way through the Napoli defence at the byeline he sent a his low drive just past the far post from a difficult angle.
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