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Alessio Romagnoli's powerful header had been Milan's reward for a strong start, but Gianluca Scamacca's excellent long-range strike and an own goal from the unlucky Simon Kjaer helped mid-table Sassuolo reach the interval with a lead.
Berardi finished off Milan with a fine goal in the 66th minute, the Italy forward showing his class to outwit Romagnoli before beating Mike Maignan.
A red card for captain Romagnoli for hauling down Gregoire Defrel with effectively a rugby tackle compounded a bad day for Milan, who began the day level on points with leaders Napoli.