AC Milan missed the chance to move into the top four in Serie A after Federico Chiesa's second-half stunner at the San Siro sealed all three points for Fiorentina.
Despite dominating for large periods, Gennaro Gattuso's side were undone after Cheisa let fly a 25-yard strike beyond Gianluigi Donnarumma - to inflict a fourth defeat of the campaign on the Rossoneri.
The hosts' Hakan Calhanoglu twice come close in the first opening, dragging a shot wide of Alban Lafont's post, before highly rated youngster Nikola Milenkovic produced a goal-line clearance to deny the Turkish midfielder a first league goal this term.
Milan continued to dominate proceedings in the second-half, with both Suso and Gonzalo Higuain unable to beat the inspired Lafont - before Chiesa's wonder strike.
In a desperate attempt to salvage a point, Gattuso's side piled forward with their best chance falling to defender Ricardo Rodriguez, but they succumbed to a damaging defeat - both to Milan and their feisty Italian manager's reign.
Key Talking Point
After a disappointing fortnight that saw the Italian giants crash out of the Europa League, Gattuso knew only a win would reprieve some rest-bite from their problems on the field.
Despite their valiant attempt to break the deadlock, they failed to create any clear cut chances after Milenkovic's goal-line intervention. Diego Laxalt and and Patrick Cutrone were summoned from the bench on the hour mark, but neither would have the desired impact - leaving former Rossoneri man Gattuso under increasing pressure.
Starting XI: Donnarumma (6), Abate (6), Zapata (6), Romagnoli (6), Rodriguez (7), Calabria (6), Mauri (7), Calhanoglu (7), Suso (8*), Castillejo (6), Higuain (6).
Substitutes: Cutrone (5), Laxalt (5), Conti (N/A).
So often the Rossoneri's most dangerous outlet this season, Suso was at the heart of their attacking intentions against a stubborn Viola outfit.
Bamboozling defenders, his near post strike was well saved by Lafont, before he created arguably Milan's best chance of the game. His pinpoint cross was met by Gonzalo Higuain, but the former Juventus striker could only look on as his shot was tipped around the post.
Despite forcing a good save from Lafont on the hour mark, Gonzalo Higuain was otherwise absent in a fruitless attacking display from the Argentinean.
Unable to add to his five league goals this season, the former Juventus man has now failed to score in his last six Serie A outings.
Starting XI: Lafont (8), Milenkovic (7), Pezzella (7), Hugo (6), Biraghi (6), Benassi (6), Veretout (7), Fernandes (6), Mirallas (5), Chiesa (7), Simeone (6).
Substitutes: Pjaca (N/A), Gerson (6).
Aiming to bounce back immediately, Milan travel to Frosinone on Boxing Day before hosting SPAL before the Serie A winter break.
Meanwhile, seventh place Fiorentina host Parma on Boxing Day, where they will look to build on this impressive, and slightly unexpected, victory.