Atalanta stunned Serie A leader Napoli by winning 3-0 at the Stadio Diego Armando Maradona on Sunday.
Mateo Retegui was surprisingly benched but Ademola Lookman netted twice in the first half to inflict what was only Napoli´s second defeat of the season, and the first since the opening day.
Retegui was brought on with 14 minutes remaining and scored in stoppage time for his 11th goal in as many matches.
Atalanta soared into second spot although it could be leapfrogged by Inter Milan if the defending champion beats Venezia later to cut the gap to Napoli to one point.
Napoli visits Inter next weekend.
Atalanta coach Gian Piero Gasperini surprisingly opted to drop Retegui. That was reportedly because he wanted a more unpredictable attack against a Napoli defence that had kept clean sheets in six of its past seven matches and only conceded five goals all season.
Gasperini instead picked Lookman and Charles De Ketelaere in attack with Mario Pasalic behind them and that paid off almost immediately as Atalanta took the lead in the 10th minute.
A cross was cleared by Napoli but only as far as the edge of the area where it was put back in and De Ketelaere nodded it back for Lookman to slot into the near bottom corner.
Napoli almost levelled immediately but Scott McTominay´s powerful effort crashed off the right post.
The Atalanta forwards combined again in the 31st as De Ketelaere raced down the right and squirmed between two players before passing across to Lookman, who slotted a hard shot into the far bottom corner.
Sead Kolasinac thought he had extended Atalanta´s lead early in the second half but the defender was offside.
Retegui was brought on for De Ketelaere late on and he volleyed a cross into the bottom right corner.
Fiorentina can move into third spot with a win at Torino later. Roma was playing at Hellas Verona.
