Juventus defender Angelo Ogbonna was sent off seven minutes from time following a second yellow card.
Sassuolo defender Alessandro Longhi's own goal looked to have secured victory for Roma. But 19-year-old Domenico Berardi leveled with the last kick of the game.
"We lacked a bit of efficiency in the second half and in football anything can happen," Roma coach Rudi Garcia said. "Congratulations to Sassuolo, who believed right up until the end.
"That's football. We should have scored a second goal and closed the match."
A double from U.S.-born forward Giuseppe Rossi helped Fiorentina beat Sampdoria 2-1.
Juventus and Napoli were level in the standings and both knew either team could take a potentially decisive step in the title race. The home side got off to the perfect start when Carlos Tevez deflected Mauricio Isla's effort for Llorente to tap home. Replays showed the Juventus striker was marginally offside.
Pepe Reina pulled off fantastic saves to deny Pogba and Leonardo Bonucci from point-blank range before Napoli started to get into the game.
Lorenzo Insigne, Gonzalo Higuain and Marek Hamsik all went close for the visitors before a trademark free kick from specialist Pirlo doubled Juve's lead in the 74th minute.
Pogba put the game beyond doubt 10 minutes from time with a stunning 25-yard (meter) strike into the top right corner.
Fiorentina moved fifth, eight points behind Roma but was forced to cling on in the final stages.
Rossi scored his first from the spot in the 11th minute after Alberto Aquilani had been brought down by Gaetano Berardi. The Italy striker then curled a spectacular effort into the top left corner to take his tally to 11 goals in 12 matches.
Manolo Gabbiadini pulled one back with 16 minutes left to set up a tense finale.
Roma's record start of 11 opening wins was halted by a draw at Torino last week. Sassuolo had started the season badly but had only lost one of its previous four matches.
Sassuolo coach Eusebio Di Francesco was given a warm welcome by the home fans. The former midfielder spent four years at Roma, winning the title in 2001.
Roma took the lead in the 19th minute when an initial effort by Alessandro Florenzi was brilliantly saved by Gianluca Pegolo, but Longhi bundled the rebound into his own net.
Adem Ljajic almost doubled Roma's lead in first-half stoppage time but Pegolo pulled off a fantastic one-handed save.
Sassuolo snatched a point when Berardi blasted in under the bar following a goalmouth scramble.
Milan dominated in Verona but failed to score. Robinho hit the outside of the post and Milan went even closer 10 minutes from time when the Brazilian again hit the woodwork and his shot rebounded to Kaka, who fired over the bar.
Riccardo Montolivo was sent off seven minutes from time following a second booking and was followed by Chievo substitute Sergio Pellissier in stoppage time.
Milan has now picked up just two points in four rounds and coach Massimiliano Allegri's job is very much at risk.
"I just talked to (club president Silvio) Berlusconi," Milan vice president Adriano Galliani said. "He agrees with me. If Robinho had scored instead of hitting the post we would be talking about a win. At least finally we didn't concede. We'll go forward with Allegri."
Lazio's misery also continued as it drew 1-1 at Parma, while high-flying Hellas Verona lost 2-0 at Genoa.
Lazio had won just one of its past six matches but seemed on course for victory when 18-year-old Keita Balde scored on his Serie A debut. However, Alessandro Lucarelli headed in the equalizer.
Genoa, which has improved since Gian Piero Gasperini took over, earned its fourth win in five matches thanks to goals from Daniele Portanova and Juraj Kucka.
Former Genoa striker Luca Toni headed a free kick off the post for Verona, which had won five of its past six games.
Elsewhere, a pair of Daniele Conti free kicks helped Cagliari to a 2-1 victory over Torino, ending a run of three successive defeats. Ciro Immobile had leveled for Torino.
Marko Livaja scored in stoppage time to snatch a 2-1 win for Atalanta against Bologna.
