Franck Kessie's penalty 17 minutes from time gave Milan a battling point in Naples after Giovanni Di Lorenzo and Dries Mertens had put the hosts deservedly ahead with an hour gone.
The draw leaves Napoli in sixth place and the final Europa League spot, but as Gennaro Gattuso's side have already qualified for the competition's group stage by winning the Italian Cup, seventh place currently offers a place into the qualifying rounds.
Milan, who are two points and one place behind Napoli, are favourites to take the third spot in the Europa, although eighth-placed Sassuolo look keen for continental football despite being four points further back.
"When you get results and put in strong performances against this kind of opponent, it really boosts your confidence," said Milan coach Stefano Pioli.
"We scored a great goal to take the lead, we fought back to equalise and a point is good".