Atalanta consigned AC Milan to their second league defeat of the season with a 3-0 win at San Siro on Saturday, but Stefano Pioli's side were still named "winter champions" in Serie A.
Cristian Romero headed home Robin Gosens' corner after 26 minutes before Josip Ilicic was brought down by Milan's Franck Kessie in the area midway through the second half.
Ilicic converted the penalty before Duvan Zapata added a third late on.
Inter Milan, who could have gone level at the top with a win, were held to a goalless draw at Udinese.
Atalanta, who become just the second side to beat AC Milan in Serie A this season.
"Today a lot of things were lacking," Milan striker Zlatan Ibrahimovic told Sky Sports.
"I don't want to make excuses, you need to go into every game to get the result. Today we didn't manage that."
Only two Serie A winter champions have failed to finish on top since the league reverted to 20 teams in 2004: Napoli in 2015-16 and 2017-18.
"We're halfway there, this means that we've done well up until now," Ibrahimovic added.
"Now is the most difficult period with a lot of games. We need to stay well physically."
Atalanta moved into the top four on 36 points, a record tally for the Bergamo club at the midway stage of a Serie A season featuring 20 teams.