AC Milan missed the chance to extend their lead at the top of Serie A standings after suffering a 3-0 loss to Atalanta.
The visitors got the breakthrough on 26 minutes through Cristian Romero's first league goal of the season.
With Milan unable to fire back in the first half, Josip Ilicic doubled Atalanta's lead from the penalty spot eight minutes of the after the break.
Mario Mandzukic came on for his Milan on 70 minutes to help rescue a result, but Duvan Zapata wrapped up the win with the third goal seven minutes later.
Inter Milan failed to take a big advantage of AC Milan's slip up as they were held to a goalless draw by Udinese on Saturday night.
Antonio Conte went with the tried and tested for this occasion , but they were unable to find the back of the net for the second league game in a row.
As a result, Inter missed the chance to move top and now sit two points behind their fierce city rivals.
Sixth-place Juventus will look to take full advantage when they face Bologna on Sunday afternoon.
