Federico Chiesa's strike 17 minutes from time earned Fiorentina their first away win of the season as AC Milan's recent struggles continued in a 1-0 loss at San Siro.
The visitors barely threatened Gianluigi Donnarumma's goal but Chiesa jinked past Davide Calabria and fired home his first goal since September to settle the contest.
The Rossoneri have now won just one of their previous six contests in Serie A and have failed to score in any of their last three.
Milan also dropped out of the top four thanks to Lazio's win earlier in the day, with Fiorentina now just two points behind them in seventh.
Milan entered this contest on the back of two 0-0 draws in Serie A and failed to find a way through in the opening 45 minutes, despite creating plenty of opportunities.
Hakan Calhanoglu spurned one of the best, firing wide in the 18th minute after a stroke of fortune saw the ball ricochet back into his path in the box following an attempted give and go with Suso.
Fiorentina goalkeeper Alban Lafont brilliantly parried away a viciously struck effort from Ricardo Rodriguez while Calhanoglu was denied by Nikola Milenkovic's clearance off the line on the stroke of half-time.
It took until the 51st minute for Fiorentina to fashion their first decent opening as Kevin Mirallas hooked way over from inside the box.
Lafont was kept busy at the other end, though, denying Suso at his near post and then preventing Gonzalo Higuain's header from creeping past him.
With just over 20 minutes to go Milan coach Gennaro Gattuso threw on Diego Laxalt and Patrick Cutrone in a bid to inject some life into his below-par outfit but it was Fiorentina that would claim the winner.
Chiesa knocked the ball inside Calabria and his right-footed attempt crashed into the corner. Lafont then tipped Rodriguez's header over as Milan fell to their fourth league loss of the season.
What does it mean? Pressure building on Gattuso
Five wins in six between September 30 and November 4 had eased the pressure on Gattuso but one win in the half a dozen since will not have pleased the Milan hierarchy. There are just two games remaining before the winter break and Gattuso will be under increasing scrutiny if Milan's form does not pick up soon.
Milan unable to find a way past Lafont
Milan did have their chances but French teenager Lafont was equal to everything thrown at him. Rodriguez in particular must be wondering how on earth he did not score as Lafont parried away an attempt destined for goal in the first half before displaying terrific agility to tip over a late header from the Swiss too.
Goal-shy Gonzalo fails to fire again
It is now eight games without a goal for Higuain and it is no surprise Milan are toiling while the Argentinian experiences a barren spell. Of more concern to Gattuso will be how ineffective Higuain was. He had just one chance across the contest and looks like a man burdened by his current struggles.
What's next?
Milan's next assignment is a trip to Frosinone, who are under new management after replacing Moreno Longo with Marco Baroni this week. Fiorentina welcome Parma to Florence on Wednesday.
