Napoli are halfway to their first Serie A title since Diego Maradona led the club to the 1990 championship.
Gonzalo Higuain scored twice as Napoli routed promoted Frosinone 5-1 on Sunday to earn the southern team the Italian league's symbolic "winter title."
Through 19 rounds of the 38-match season, Napoli hold a two-point lead over Inter Milan, which was beaten 1-0 by Sassuolo earlier with a last-minute penalty to surrender the lead.
The winter champions have gone on to win Serie A nearly seven out of every 10 seasons.
The last time Napoli earned the mid-term honor came in 1990.
Raul Albiol put Napoli ahead from close range midway through the first half when Frosinone goalkeeper Massimo Zappino appeared blinded by the sun.
Higuain added a penalty on the half-hour mark, Marek Hamsik then scored his third goal in three matches with an impressive solo move after the break, before Higuain dribbled past three defenders to score his league-best 18th.
Substitute Manolo Gabbiadini added another for Napoli with a shot from beyond the area and Paolo Sammarco then pulled one back for Frosinone, a club located midway between Rome and Naples.
Earlier, Domenico Berardi converted the decisive spot kick for Sassuolo five minutes into added time after Inter defender Joao Miranda was shown a yellow card for a foul on Gregoire Defrel.
Fiorentina is third, three points behind, after losing 3-1 to Lazio on Saturday.
Later, fourth-place Juventus was visiting Sampdoria seeking its ninth consecutive win and a chance to move ahead of Inter into second.
Also Sunday, it was: Atalanta 0, Genoa 2; Bologna 0, Chievo Verona 1; Torino 0, Empoli 1; and Hellas Verona 0, Palermo 1.
In an entertaining match at the San Siro, both Inter and Sassuolo produced numerous chances and goalkeepers Samir Handanovic and Andrea Consigli were superb.
Inter threatened early with Mauro Icardi and Adem Ljajic, then Simone Missiroli and Nicola Sansone were dangerous for Sassuolo.
Handanovic tipped a free kick from Sansone over the bar and Consigli made an extraordinary diving save to swat away a shot from Ljajic before the break.
Inter substitute Rodrigo Palacio had a clear look in the 79th but his effort was cleared off the line by defender Federico Peluso.
Miranda's decisive foul came after an error by fellow center back Jeison Murillo.
Berardi then calmly beat Handanovic, improving to a perfect 6-for-6 on penalties this season.
Having been beaten by Lazio before the holiday break, Inter has now lost two consecutive home matches in Serie A for the first time since May 2013.
Sassuolo is sixth, 10 points behind Napoli.