Sassuolo stun floundering Juventus
The defeat leaves Juventus below the European spots after the Turin giants were docked 15 points for illicit transfer activity in January.
Runaway leaders Napoli drew on Saturday as Italy awaits an explosive party in Naples where Napoli look certain to clinch a first title since the days of Diego Maradona.
Italy's champions-elect have had their imposing league lead cut to 14 points after a second-string XI could only manage a goalless stalemate with Verona and second-placed Lazio won at Spezia on Friday.
On Sunday Juventus, dressed in a pink and blue strip, started with Arkadiusz Milik and Dusan Vlahovic up front in a drab first half.
Shortly after the break Drefel pounced on an error, pivoted, and fired a low hard shot past Mattia Perin in the Juve goal.
Coach Massimiliano Allegri then made a swathe of substitutions bringing on Paul Pogba and Angel di Maria to no avail.
Perin was in goal as regular 'keeper Wojciech Szczesny was benched after being subbed with chest pains in midweek.
Juve beat Sporting 1-0 in the Europa League in midweek, and here showed signs of fatugue.
Juventus are also waiting to see if they and 12 other current and former club figures will face trial for alleged financial crimes in May.