Massimilano Allegri's 200th match in charge of Juventus has ended in defeat as Sassuolo scored late to snatch a 2-1 Serie A victory in Turin.
Maxime Lopez struck in the fifth minute of stoppage time to hand Sassuolo a first-ever victory at Juventus.
American Weston McKennie had cancelled out Davide Frattesi's first-half opener.
Juventus are seventh, 13 points behind Serie A leaders AC Milan after 10 matches.
Champions Inter Milan closed the gap to their city rivals to seven points with a 2-0 win at 10-man Empoli.
Sassuolo moved up to ninth, just a point behind Juventus.
After a rocky start to Allegri's second spell in charge, Juventus had appeared to be back on track to be unbeaten in 10 matches in all competitions.
However, Sassuolo took the lead a minute from halftime when Frattesi fired Gregoire Defrel's through ball into the bottom left corner of the goal.
The Bianconeri got their equaliser when McKennie headed a Paulo Dybala free kick into the net in the 76th minute.
Juventus pushed hard for the winner and all the Bianconeri players were at the other end, leaving them wide open at the back when Lopez sprinted onto a long ball and managed to evade McKennie's last-ditch tackle and win the match with a chipped effort.
"There are games that if you don't win you can't afford to lose," Allegri told DAZN.
"A draw would have meant we extended the unbeaten run and we would not have lost twice at home (this season) already.
"We have to be realistic. We have to play in another way and be more careful, especially if you meet teams like Sassuolo."
Goals from Danilo D'Ambrosio and Federico Dimarco ensure third-placed Inter kept up the pressure on the top two: Milan and Napoli.
Inter took the lead in the 34th when D'Ambrosio headed in a delightful chipped pass from Alexis Sanchez and the match swayed further in their favour seven minutes into the second half when Empoli midfielder Samuele Ricci was shown a straight red card for a horrendous tackle on Nicolo Barella.
Dimarco doubled Inter's lead in the 66th when he fired in a pinpoint cross to the back post from Lautaro Martinez.
Meanwhile, Roma, Atalanta and Lazio all won to keep in sight of the top spots.
Roma remained fourth, two points behind Inter, after they came from behind to win 2-1 at relegation-threatened Cagliari.
Atalanta are a point further back and also had to recover from conceding first in a 3-1 victory at Sampdoria.
Lazio beat Fiorentina 1-0 to move above their opponents and Juventus into sixth spot.
Elsewhere, Udinese drew 1-1 with Verona.
Australian Associated Press