Atalanta beat AC Milan 3-0 at the San Siro on Saturday in Serie A, but Stefano Pioli's side remains top of the table.
A Cristian Romero header put the visitors ahead at halftime before a Josip Ilicic penalty and Duvan Zapata strike completed a dominant victory for the Bergamo side.
Atalanta had the chances to win by an even wider margin as an out-of-sorts Milan put in one of their worst performances of the season.
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The home side failed to muster a shot on target until debutant Mario Mandzukic drew a good save from Pierluigi Gollini in the 71st minute with his first touch after coming off the bench.
Milan nevertheless claim the title of winter champions as they top the standings at the halfway point of the season with 43 points, two ahead of Inter Milan after their 0-0 draw at Udinese.
Atalanta moved into the top four on 36 points, returning to form in style after back-to-back draws against Genoa and Udinese.
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