Lorenzo Insigne washed away his World Cup woes by starring in Napoli's 2-1 victory over struggling but spirited AC Milan on Saturday that saw the Serie A leaders go four points clear.

Maurizio Sarri's team are also five ahead of third-placed Roma, who are level on 30 with Inter Milan after beating Lazio 2-1 in a spectacular Rome derby earlier thanks to goals from Diego Perotti and Radja Nainggolan.

Milan manager Vincenzo Montella remains after pressure despite a strong second half performance as his side stay seventh on just 19 points, 11 points away from the Champions League placings, after their sixth defeat in 13 games.

Winger Insigne, ignored by former Italy coach Gian Piero Ventura during the national team's play-off disaster against Sweden, helped extend Napoli's lead over Juventus ahead of the champions visit to in-form Sampdoria on Sunday with a 33rd-minute finish that was as smart as it was controversial.

The 26-year-old was wrongly flagged offside after bursting through to meet fellow Italy outcast Jorginho's precision through ball and shoot past Gianluigi Donnarumma, and it looked like the goal would be ruled out.

However, despite having appeared to have already blown his whistle for offside, referee Daniele Doveri consulted the video assistant referee, who confirmed that Milan defender Alessio Romagnoli had played Insigne onside.

Insigne could have ended the half with more as Milan almost buckled completely in the face of intense Napoli pressure.

He tested Donnarumma with a superb curling shot on the stroke of half time and hit the post just before a late offside flag was wrongly raised.

The winger also laid on a great chance for Dries Mertens just after the break following a neat exchange of passes with the Belgian, who could only fire straight at Donnarumma with the goal at his mercy.

It was substitute Piotr Zielinski who sealed the three points 25 minutes later, collecting Dries Mertens' brilliant clipped pass and drilling home to give Napoli a cushion that meant Milan's Romagnoli's stunning stoppage volley was nothing more than a consolation.