Luciano Spalletti's side looked set to maintain their three-point lead at the top of the league following second-half strikes from Dries Mertens and Fabian Ruiz.
But a superb finish from Gianluca Scamacca with 19 minutes remaining put the hosts back in the game and Gian Marco Ferrari snatched a draw in the final minute with a bullet header.
Sassuolo thought they had won it in a franctic finale in which Napoli coach Luciano Spalletti was sent off when Gregiore Defrel slotted home the third. But with the home players and fans celebrating wildly, the referee was called to check a Domenico Berardi foul on Amir Rrahmani and the goal was ruled out.
To make matters worse for Napoli, who host high-flying Atalanta on Saturday, they lost both scorers and defensive lynchpin Kalidou Koulibaly to injury.
December 1, 2021
AC Milan are now a point back in second after strolling to a 3-0 win at Genoa while Inter Milan are a further point behind in third following their 2-0 home victory over Spezia.
Junior Messias continued his incredible rise with a brilliantly taken brace either side of half-time at the Stadio Luigi Ferraris after Zlatan Ibrahimovic had curled in a perfect free-kick to give Milan a 10th minute lead.
Brazilian Messias, a 30-year-old former fridge deliveryman who is only in his second ever Serie A season, looped in a distance header in first half stoppage-time to double the away side's lead before rolling home a delicate finish in the 61st minute to seal the points.
His double comes after he struck the winning goal at Atletico Madrid which keeps alive Milan's hopes of making the Champions League last 16 ahead of the visit of Liverpool next week.
December 1, 2021
Bologna are eighth, level on 24 points with Juventus and Fiorentina after Mattias Svanberg fired them to a 1-0 win over Jose Mourinho's Roma.
Swede Svanberg, who was reportedly being watched at the Stadio Renato Dall'Ara by scouts from Manchester City, struck a beautiful winner from distance to move Sinisa Mihajlovic's side to within one point of fifth-placed Roma.
The result is also great news for Atalanta, who after their win over Venezia on Tuesday are six points away from their nearest challenger for fourth and the final Champions League spot.