Milan failed to move back within three points of Serie A leaders Napoli as goals from Koffi Djidji and Aleksey Miranchuk condemned the Rossoneri to a surprise 2-1 loss to Torino.
Miranchuk followed up Djidji's glancing header with a cultured left-footed finish as Torino scored twice in two frantic first-half minutes to stun Milan at the Stadio Olimpico Grande Torino.
Stefano Pioli's men travelled to Turin looking to stay within touching distance of the Serie A summit, but were uncharacteristically lethargic for long periods on Sunday.
Junior Messias was gifted a goal following a bizarre mix-up at the back, but Milan failed to build on that strike late on as Torino clung on for a huge win.
Rafael Leao missed two golden chances in the first six minutes, sending a wild volley over after breaking in behind before mishitting his shot after being teed up by Brahim Diaz.
Torino punished Milan for their wastefulness 10 minutes before the break, as Djidji nodded Valentino Lazaro's deep free-kick in off the right-hand post.
Things quickly went from bad to worse for the champions as Miranchuk latched onto Nikola Vlasic's knock-down before shifting the ball onto his left foot and finishing coolly across Tatarusanu.
Sergino Dest fired over from range as Milan attempted to lift the tempo after the break, before Messias ensured a tense finish when he curled home following a clash between Vanja Milinkovic-Savic and Perr Schuurs.
Torino boss Ivan Juric was dismissed for berating referee Rosario Abisso following that goal, but his team showed greater composure to see out the closing stages in relative comfort.
What does it mean? Rossoneri fall short
Although the likes of Napoli and Atalanta have made headlines in Serie A by emerging as surprise pacesetters, Milan had begun their title defence in solid fashion, stretching their unbeaten away run to 17 league games ahead of Sunday's contest (W12 D5).
Milan had also kept five consecutive clean sheets in league meetings with Torino prior to this game, but they saw both of those records fall by the wayside after a quickfire double from their hosts, who richly deserved the win following a disciplined performance.
Djidji ends barren run
To say Torino have struggled against Milan in recent meetings would be an understatement: Djidji's opener ended a 499-minute run in which Torino had failed to find the net against Milan in Serie A meetings.
Their previous goal against the Rossoneri in the competition was scored by Andrea Belotti in September 2019, and prior to Sunday's game, no player in Torino's current squad had scored a league goal against Milan.
Leao kept quiet
Leao has emerged as the most obvious goal threat in an exciting Milan team, having recorded nine league goal contributions (five goals, four assists) ahead of the trip to Turin.
However, the Portugal attacker was hauled off at the break after a disappointing first half in which he squandered chances amounting to 0.51 expected goals (xG) and failed to hit the target with any of his four attempts.
What's next?
Milan host Salzburg in the Champions League on Wednesday, needing to avoid defeat to reach the last 16. Torino, meanwhile, go to Bologna in Serie A next Sunday.