AC Milan's Rafael Leao scores the first goal for his team on Sunday in Reggio Emilia, Italy. Photo: VCG

AC Milan forward Rafael Leao scored the fastest-ever goal in Serie A history on Sunday, netting after just six seconds in a 2-1 win at Sassuolo which kept his side top of the table ahead of city rivals Inter Milan.

Milan, chasing a first Serie A title since 2011, launched straight onto the attack from kickoff, with Turkish midfielder Hakan ­Calhanoglu setting up Leao to slot home the rapid opener.

The previous fastest goal in the Italian top flight had been Paolo Poggi's for Piacenza against Fiorentina in December 2001 after eight seconds.

Milan have a one-point advantage on Antonio Conte's Inter who beat Spezia 2-1, with champions Juventus four points behind the leaders in third after a 4-0 win over Parma on Saturday.

"We have four or five kickoff patterns. [Leao] was prepared," said Milan coach Stefano Pioli whose side had been held 2-2 in both their last two league games against Parma and Genoa. "Winning this match counts for a lot psychologically."

Portuguese Leao, who usually plays on the left wing, started at center-forward with star striker Zlatan Ibrahimovic still out with a leg muscle problem and his replacement Ante Rebic also injured.

It was the 21-year-old's third league goal this season and fourth in all competitions.

Calhanoglu thought he had scored a second on nine minutes when he put home a Leao cross but the goal was disallowed for offside after a VAR review.

However, Alexis Saelemaekers doubled Milan's lead in the 26th minute following good work from Theo Hernandez.

Domenico Berardi pulled a goal back late for Sassuolo, who sit sixth after the defeat.

At the San Siro, Inter Milan extended their winning league run to six matches with Achraf Hakimi getting the first goal five minutes after the break.

Romelu Lukaku slotted in a penalty on 71 minutes for his 11th league goal this season after a M'Bala Nzola handball.

Roberto Piccoli got a goal back for Spezia, who are just above the relegation zone after extending their winless run to six games, deep into injury time.

Atalanta came from behind to crush Roma 4-1 in Bergamo despite playing without captain Papu Gomez who has fallen out with coach Gian Piero Gasperini.

Edin Dzeko scored after just three minutes, with the Roma captain finishing from a Henrikh Mkhitaryan cross.

But Josip Ilicic came off the bench after the break to inspire the Atalanta recovery, setting up Duvan Zapata for the equalizer just before the hour mark and crossing for Robin Gosens to head in 10 minutes later.

Colombian Luis Muriel scored the third a minute after coming off the bench on 72 minutes, before Ilicic got his first league goal of the season with an impressive solo run five minutes from time.

Atalanta, with a game in hand, are seventh in the table, 10 points off AC Milan.