Atalanta caused a huge upset, beating Napoli when down to 10 men thanks to a Mattia Caldara brace to move within three points of third place.
It is also the first time Maurizio Sarri's men have failed to score at home in Serie A since December 2015. They lost both home and away to Atalanta this season, 3-0 on aggregate.
This was a surprisingly important clash between third and joint-fourth, as the Orobici had already beaten Napoli earlier this season with an Andrea Petagna strike. The hosts rested Kalidou Koulibaly and missed Lorenzo Tonelli and Allan through injury.
Atalanta had conquered the San Paolo just once in their last 13 competitive visits, 3-1 in April 2012, along with five draws and seven defeats.
It was a sparkling start under the driving rain, as Andrea Conti’s scuffed volley was smothered and a Lorenzo Insigne curler bounced off the top of the crossbar.
Rafael Toloi acrobatically cut out a Marek Hamsik through ball and awkwardly landed on his head, but fortunately escaped only a little dazed.
Atalanta certainly weren’t sitting back and after Jasmin Kurtic dribbled down the right to force a corner, that set piece bounced off defenders for Mattia Caldara to nod in from close range.
Piotr Zielinski blasted over and Insigne’s effort was too central to beat Etrit Berisha, but Leonardo Spinazzola’s cross-shot found nobody ready to tap in after bursting past Elseid Hysaj.
Dries Mertens left Caldara standing with his sensational first touch only to then turn the angled drive inches past the post.
On the stroke of half-time, a desperate Berisha save fingertipped Mertens’ free kick on to the crossbar, as it was flying into the near top corner.
Within 45 seconds of the restart it should’ve been 2-0 when Nikola Maksimovic slipped trying to intercept a Toloi through ball, but Petagna fired straight at Reina in a one-on-one situation.
Atalanta continued to push with a series of corners, though Kessie went through the back of Amadou Diawara and will be suspended against Fiorentina.
Arkadiusz Milik came on, moving Mertens to a wider role, and the Belgian had a snapshot smothered. Atalanta youngster Kessie picked up his second yellow card in just a few minutes, as he was late stopping Insigne sparking a counter from their corner.
However, despite going down to 10 men, Atalanta doubled their lead moments later. Spinazzola’s dinked cross from the left found Caldara for an acrobatic volley from 12 yards, raising his right foot high to steer it into the near bottom corner, bouncing just in front of Reina. It was the first brace for the defender – who has already been bought by Juventus for 2018.
Napoli fought back, Zielinski’s daisy-cutter inches wide and nobody to meet Faouzi Ghoulam’s pull-back. Above all, Ghoulam’s cross found Jose Callejon for a free header, which he somehow turned wide from six yards.
Berisha punched away a Mertens curler, while Napoli laid siege on the Atalanta goal in the final stages, but the 10-man minnows held on.
See the latest Serie A predictions and betting tips with Eurotips.co.uk
Sent off: Kessie 67 (A)