Gonzalo Higuain scored twice, taking his tally to 18, for Napoli as the lead changed hands for the 10th time in 19 rounds of games this season.
Napoli, with 41 points, are two clear of Inter and lead at the halfway mark, making them so-called "winter champions", for the first time since the 1989/90 season, the last time they won the title.
Titleholders Juventus pulled level on 39 points with Inter after Paul Pogba and Sami Khedira scored to give them a 2-1 win at Sampodoria, their ninth league win on the trot.
Antonio Cassano replied for Samp, who had Niklas Moisander sent off for a wild challenge in stoppage time.
Inter dominated their match at soggy San Siro but were repeatedly foiled by the visitors' goalkeeper Andrea Consigli.
Berardi sent Inter goalkeeper Samir Handanovic the wrong way to complete an extraordinary win for Sassuolo, who were beaten 7-0 in the same fixture last season, as Inter coach Roberto Mancini watched in disbelief.
The penalty was awarded after Inter defender Jeisson Murillo mistimed a header, then team mate Miranda clumsily bundled over Gregoire Defrel as the hosts made a pig's ear of dealing with an innocuous Sassuolo attack.
"That’s football," said Mancini. "We created lots of chances but Consigli was fantastic and we just couldn’t get the goal."
Inter missed a flurry of early chances and when Adem Ljajic finally managed to get a curling effort on target just before halftime, his effort was brilliantly tipped away by Consigli.
Consigli again denied the Serbian after the break, then did well to parry away a low drive from Geoffrey Kondogbia as he kept sixth-placed Sassuolo in the game.
In an end-to-end game, Inter forward Rodrigo Palacio had a shot cleared off the line while, at the other end, Handanovic made a superb reflex save at close range to stop an acrobatic volley from Antonio Floro Flores.
Consigli again denied Ljajic and turned away a curling effort from substitute Stevan Jovetic before Berardi struck for Sassuolo.
Raul Albiol turned the ball in at the far post before Higuain won and converted a penalty to put Napoli 2-0 ahead at Frosinone after half an hour.
Marek Hamsik and Higuain scored twice in a minute after the break, the Argentine's fourth brace in the last six games, and Manolo Gabbiandini added the fifth goal.
Unheralded Empoli went seventh, above AC Milan, after winning 1-0 at Torino with a second-half goal by Massimo Maccarone.
There were also 1-0 away wins for Chievo, at Roberto Donadoni's Bologna, and Palermo at bottom club Verona, who remained winless. Genoa's 2-0 win at Atalanta took them four points clear of the relegation zone.
(Writing by Brian Homewood in Berne; editing by Clare Fallon and Justin Palmer)
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