Carlos Carmona's goal in the first half proves to be the difference between Atalanta and Napoli on Wednesday.
Napoli have suffered their second defeat in Serie A this season as they went down 1-0 to Atalanta at Stadio Atleti Azzurri d'Italia.
The visitors from Naples had the better chances in the opening exchanges, with Marek Hamsik and Goran Pandev combining to release Andrea Dossena, who failed to get a proper kick on the ball.
Lorenzo Insigne then saw his volley saved by Atalanta goalkeeper Andrea Consigli.
Much against the run of play the hosts broke the deadlock in the 19th minute, when German Denis found Carlos Carmona just outside the box and the Chilean midfielder volleyed past Morgan de Sanctis.
Four minutes later Denis thought that he had doubled his side's advantage only to see his effort get ruled out for an offside infringement.
Davide Brivido smashed wide a Denis cross before Hamsik saw his volley saved at the other end.
Both sides continued to exchange chances in the second half, Insigne and Giandomenico Mesto going close for Napoli and Maximiliano Moralez for Atalanta.
However, there were no more goals as Atalanta held on for a narrow win.
The result sees Napoli slip to third in the standings behind Juventus and Inter Milan.