AC Milan's title hopes took an almost fatal blow on Sunday evening as they were beaten 1-0 at home by Napoli.
A second-half goal from Matteo Politano proved to be the difference at San Siro as Milan now fall nine points behind league leaders Inter.
The first half was a reasonably tight affair, with no clear-cut chances for either side.
Piotr Zielinski forced a save out of Gianluigi Donnarumma and also fired wide, while Rafael Leao was wasteful down the other end.
But it was Napoli who broke the deadlock just four minutes into the second half as Piotr Zielinski carved open the Milan defence and Politano slotted home with his weaker right foot.
SERIE A TABLE: Inter go nine points clear
There were chances for both sides after Politano's goal as Fabian Ruiz, Lorenzo Insigne and Victor Osimhen threatened for Napoli, while Leao and Ante Rebic had chances for Milan.
There was a late VAR review which could have given Milan a penalty for a foul on Theo Hernandez, but the referee opted against it after watching the pitchside monitor.
In injury time, Rebic was sent off for dissent to compound Milan's misery.
Earlier in the day, Inter won 2-1 at Torino to take their winning streak to eight games.
Meanwhile, Juventus are now only one point behind second-placed Milan - with a game in hand - after a Cristiano Ronaldo hat-trick defeated Cagliari 3-1 in Sardegna.
Red card for Ante Rebić.
Yellow card for Theo Hernández.
Substitute in, Mário Rui.
Substitute in, Tiemoué Bakayoko.
Substitute out, Matteo Politano.
Substitute out, Diego Demme.
Substitute in, Jens Hauge.
Substitute out, Rafael Leão.
Substitute in, Eljif Elmas.
Substitute out, Piotr Zieliński.
Yellow card for Giovanni Di Lorenzo.
Substitute in, Soualiho Meïté.
Substitute out, Franck Kessie.
Substitute in, Ante Rebić.
Substitute in, Brahim Diaz.
Substitute in, Alexis Saelemaekers.
Substitute out, Rade Krunić.
Substitute out, Hakan Çalhanoğlu.
Substitute out, Samu Castillejo.
Substitute in, Victor Osimhen.
Substitute out, Dries Mertens.
Assist given by Piotr Zieliński.
Goal by Matteo Politano.
Yellow card for Nikola Maksimović.