Franck Kessie scored twice to give AC Milan a 2-1 win in windy conditions at Cagliari as both teams had a player sent off.
Lazio moved third with a thumping 5-1 win over Chievo in Rome that saw rising star Sergej Milinkovic-Savic bag a fine brace either side of half-time to put Lazio two up, after Chievo's Manuel Pucciarelli had equalised Luis Alberto's 23rd-minute long-range opener for the hosts.
Roma, who also have a game in hand, are fifth with 40 points.
Second-placed Juventus host 15th-placed Genoa in a 19:45 GMT kick-off on Monday, when the champions can narrow the gap to one point with a win. "I'm very happy for us and our supporters".
Uruguayan Vecino met a whipped Marcelo Brozovic cross to beat an outstanding Alisson, who had made two superb second-half saves from Mauro Icardi, and keep Luciano Spalletti's side in the top four, level on 43 points with third-placed Lazio and three ahead of Roma.
Belgian global Mertens' 65th-minute strike ended a league dry spell that had dated back to October 29, getting behind the defence and flashing a shot past Atalanta goalkeeper Etrit Berisha.
Napoli had the better of the match and could have gone ahead early in the second half.
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Chievo were awarded a penalty in first-half stoppage time for a foul by Senad Lulic on Mariusz Stepinksi but the referee changed his mind after consulting the video replay system (VAR).
Immobile, who has scored 20 league goals this season, limped off shortly afterwards.
Kessie levelled from a penalty kick in the 35th minute and the Ivorian struck again six minutes later. Milan defender Ricardo Rodriguez was sent off in the 81st minute for two yellow cards, both for fouls on Paolo Farago.
Quagliarella opened his account against Fiorentina after half an hour, and fired home the second after 60 minutes.
Carlos Sanchez pulled a goal back for La Viola with a header 10 minutes from time, but it was scant consolation.
Bottom club Benevento's mini-revival came to a crashing halt with a 3-0 beating at Bologna, who won thanks to goals from Mattia Destro, Sebastien De Maio and Blerim Dzemaili.
The games between Sassuolo and Torino, and Udinese and SPAL, ended 1-1.