AC Milan's Hakan Calhanoglu (right) fights for the ball with Sampdoria's Morten Thorsby on Sunday in Genoa, Italy. Photo: VCG

AC Milan kept hold of their five-point lead at the top of Serie A on Sunday after a hard-fought 2-1 win at Sampdoria, while Napoli moved third after strolling to victory over bottom side Crotone.

Franck Kessie's penalty on the stroke of halftime and Samu Castillejo's 77th-minute strike were just enough for the seven-time European champions to move up to 26 points and keep second-placed local ­rivals ­Inter Milan at arm's length.

Coach Stefano Pioli hailed a dogged display from a team missing injured talisman Zlatan Ibrahimovic, defender Simon Kjaer and midfielder Ismael Bennacer.

"This is a team that makes the most of every situation, even the most negative, to show what they can do," Pioli said to Sky Sport Italia.

"I keep being surprised by the maturity of the players... They never get downhearted by bad results or let good ones go to their heads."

Cote d'Ivoire midfielder Kessie coolly struck home his spot kick after Jakub Jankto unluckily handled while challenging for the ball with Theo Hernandez.

Sandro Tonali hit the post for ­Milan just after the break but the away side were on the back foot for much of the second half until substitute Castillejo doubled their lead with his first touch, set up by a fine pull-back from Ante Rebic.

Napoli are six points back in third, level with champions Juventus, after goals from Lorenzo Insigne, Hirving Lozano, Diego Demme and Andrea Petagna earned the away side a 4-0 win at Crotone.

Insigne opened the scoring with a trademark curling strike after cutting in from the left with half an hour gone, and the task became simpler for the away side five minutes after the break when Jacopo Petriccione was sent off for a late, high challenge on Demme.

Italy international Insigne then laid on Lozano for the second with a superb long pass that left the Mexican with a simple finish, and Demme made sure of the result with a drilled low strike 14 minutes from the end following a layoff from substitute Mertens.

Belgium forward Mertens then supplied the pass that allowed Petagna to hammer home the fourth in stoppage time.

Sixth-placed Roma meanwhile are now eight points off the lead after their goalless home draw with Sassuolo as both teams had goals ruled out by VAR and the hosts played the second half with 10 men.

Pedro was sent off for a second bookable offense five minutes before the break. His coach Paulo Fonseca followed him to the stands at the end of the first half after Henrikh Mkhitaryan's goal was ruled out for a foul by Edin Dzeko following a VAR check.

The hosts dominated after the break despite being outnumbered and were unlucky to not take the lead when Leonardo Spinazzola's cross was guided onto the post by Sassuolo defender Gian Marco Ferrari, under pressure from Dzeko.