Last season's runners-up Milan were hosted by Fiorentina and lost 1-2 in the seventh round of the current Italian Serie A season.
French midfielder Yacine Adli and Icelandic striker Albert Gudmundsson scored for the hosts.
And Fiorentina's newcomer, Spanish goalkeeper David de Gea saved two penalties.
American striker Christian Pulisic netted the only goal of Milan.
Milan are sixth, with 11 points, and Fiorentina are eleventh, with 10 points, in the current Serie A standings.
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