Milan is 13 points behind in a tie for 12th with promoted Livorno.
Napoli also entered the weekend with questions over its form following a 2-0 midweek defeat to Arsenal in the Champions League.
"We knew this was a difficult match after the loss to Arsenal," Napoli manager Rafa Benitez said. "We showed the character that I was talking about during the week."
On Saturday, Francesco Totti scored twice as Roma won 3-0 at Inter Milan to extend its winning start to seven matches. The Giallorossi have scored 20 goals and conceded just one.
In Sunday's other matches, it was: Bologna 1, Hellas Verona 4; Catania 1, Genoa 1; Lazio 0, Fiorentina 0; Parma 3, Sassuolo 1; Sampdoria 2, Torino 2; and Udinese 2, Cagliari 0.
In Turin, Milan midfielder Sulley Muntari took advantage of an errant shot from Antonio Nocerino to deflect the ball in with one touch in the match's first chance.
Andrea Pirlo equalized in the 15th with a curling free kick that appeared to surprise Milan goalkeeper Christian Abbiati. Out of respect for his former club, Pirlo did not celebrate.
Milan defender Philippe Mexes punched Juventus defender Giorgio Chiellini in the 50th as the players jockeyed for position inside the area. The referee did not see what happened but TV replays will likely be used to hand Mexes a lengthy ban.
Sebastian Giovinco gave Juve the lead in the 69th, dribbling past two defenders before beating Abbiati with a precise shot two minutes after he came on for Fabio Quagliarella.
As luck would have it, Mexes was sent off for his second yellow card in the 74th and Chiellini scored in the 75th with a rebound of another free kick from Pirlo that bounced off the crossbar.
Muntari pulled one back for Milan in the 90th with a long shot that was deflected by Juventus defender Leonardo Bonucci, leaving goalkeeper Gianluigi Buffon fuming.
Milan, which was without the suspended Mario Balotelli, nearly equalized in added time but Cristian Zapata's header from a corner sailed high.
In Naples, Goran Pandev put the hosts ahead three minutes in, Gokhan Inler doubled the lead in the 26th, Jose Callejon added another in the 54th and Marek Hamsik scored his fifth of the season in the 83rd.
Pandev finished off a counterattack after getting set up by Belgium midfielder Dries Mertens and Inler found the target with a long and powerful shot that Livorno goalkeeper Francesco Bardi should have stopped.
Callejon scored from within the area after getting set up by Pandev and Hamsik finished off another swift counterattack that followed a Livorno corner by knocking in a rebound.
Hellas Verona, back in the top division for the first time in 11 years, moved into fifth place, eight points behind Roma, after getting goals from Fabrizio Cacciatore, Juan Manuel Iturbe, former Italy forward Luca Toni and Jorginho.
The start of the match was delayed for 20 minutes because Verona's bus was held up by fan clashes outside the stadium.
Antonio Cassano, another former Italy forward, scored one goal and set up two others as 10-man Parma won a regional derby.
A minute of silence was observed before each match to mourn the 100-plus African migrants killed in a shipwreck off the Italian island of Lampedusa on Thursday.
