Inter Milan missed the chance to cut Napoli's huge lead at the top of Serie A lead on Friday after suffering a shock 2-1 defeat to relegation-threatened Spezia.

Goals from Daniel Maldini and M'Bala Nzola either side of a Romelu Lukaku penalty handed fourth-from-bottom Spezia a crucial victory in their bid to avoid the drop.

But the defeat was another bodyblow to Inter's faint hopes of overhauling Napoli at the top of the table.

Napoli are now 15 points clear of second-placed Inter at the top of the table and have played one game less.

Inter paid the price for failing to take the chances which came their way, with Argentinian forward Lautaro Martinez having a penalty saved by Spezia goalkeeper Bartlomiej Dragowski after 15 minutes.

Martinez did find the net early in the second half but Lukaku was offside.

Spezia took the lead after 55 minutes when Maldini -- the son of AC Milan legend Paolo Maldini -- swept home a low shot after good work by Nzola down the right.

Inter got back on level terms with seven minutes remaining however after Denzel Dumfries won a penalty and Lukaku converted.

Inter's joy was short-lived though, with Dumfries upending Viktor Kovalenko at the other end four minutes later to concede a penalty.

Nzola converted coolly to give Spezia its first victory in Serie A against Inter at the sixth attempt.

Inter's second consecutive away defeat means Napoli can go 18 points clear with a victory over sixth-placed Atalanta on Saturday.

Spezia stayed one spot above the relegation places but stretched their advantage of Verona, immediately below them to six points.