The 19-year-old Italian -- who became the youngest player to reach 100 Serie A appearances on Sunday -- dived to keep out Arkadiusz Milik's goal-bound effort two minutes into added-on time as Napoli
The 19-year-old Italian -- who became the youngest player to reach 100 Serie A appearances on Sunday -- dived to keep out Arkadiusz Milik's goal-bound effort two minutes into added-on time as Napoli lost further ground on champions Juventus.
Napoli -- bidding for an elusive third title after those won in 1987 and 1990 -- now have 78 points from 32 games. Six-time consecutive champions Juventus, on 81, can pull six points clear with a win against Sampdoria later Sunday.
"It's useless to think about Juventus or we won't win," said Napoli midfielder Jorginho.
"I'm sorry for the draw, we had our chances but we did not manage to score, but let's look ahead because there are still other matches."
In an intense game at a sold-out San Siro, AC Milan overcame the absences of captain Leonardo Bonucci (suspended) and centre-back Alessio Romagnoli (injured) to peg the visitors back.
Napoli goalkeeper Pepe Reina -- reported to be heading for AC Milan on a free transfer next June -- was tested early, diving low to clear Giacomo Bonaventura's effort.
Milan midfielder Suso threatened after 11 minutes with a cross as towering defender Kalidou Koulibaly deflected the ball away to bounce off Nikola Kalinic's head.
Milan defender Mateo Musacchio thought he had scored six minutes before the break firing in a rebound after Reina had blocked a Hakan Calhanoglu shot, but was ruled offside.
After the interval Napoli captain Marek Hamsik was at the centre of the action, setting up Lorenzo Insigne on the left and launching himself into the area but to no avail.
And Milik finally looked set to break the deadlock in stoppages when he picked up a cross, controlled and fired towards goal but Donnarumma raced forward and got low to tip wide.
It condemned Napoli to a third draw in their past five games.
"Apart from his late save I like the way Donnarumma played with great calm," said coach Gennaro Gattuso.
"I thought his overall performance was excellent and he showed no fear under pressure."
AC Milan remain sixth and within the Europa League places but look too far behind for the Champions League with six games to play.
"As long as we're still mathematically in contention, we believe. But there's a lot of points between us and fourth place," said Gattuso.
Inter Milan missed the chance to get back into the Champions League contention with a goalless draw at Atalanta on Saturday.
Juventus and Napoli looked locked into the first two Champions League spots with three teams fighting for the two remaining with Inter, Roma and Lazio now all on 60 points.
Roma and Lazio go head to head in the Roman derby later Sunday.
Fiorentina are two points behind Milan in seventh after a goalless draw at home against struggling SPAL ended the Tuscan side's six-game winning streak.
Meanwhile, Bologna won 2-0 at home against Verona with goals from Simone Verdi and Adam Nagy assuring their top flight survival next season.
Verona are just ahead of rock-bottom Benevento, who grabbed a 2-2 draw at Sassuolo, a double from Malian striker Cheick Diabate taking his tally to seven goals in six matches.