While the Viola eventually scraped past modest Conference League opponents, Milan could not build on their improved domestic form, suffering a second Champions League defeat in two games.
Match preview
Having reached the final in both of the past two years, Fiorentina were overwhelming favourites to see off Welsh outsiders The New Saints when kicking off their latest Conference League campaign on Thursday; however, it took over an hour to break TNS down.
Yacine Adli finally started the scoring on 65 minutes, before Moise Kean effectively made the points safe soon after, turning a rebound home from close range.
It had previously taken a penalty shootout to edge past Puskas Academy in the playoffs, and Fiorentina's league form has been inconsistent since Rafaelle Palladino took over from former boss Vincenzo Italiano.
The Viola had failed to win any of their first four Serie A fixtures before beating Lazio deep into September, and a dull derby draw with Tuscan rivals Empoli led to more frustration last weekend.
A goalless stalemate at Stadio Castellani left Fiorentina occupying 11th place in the standings, averaging just over one point per game.
Nonetheless, they have been tough to beat on home soil for some time now, losing only one of their last 11 league games in Florence - against none other than Milan, back in March.
Including that 2-1 victory, which completed the double over Fiorentina last season, Milan have won six of their last eight Serie A matches against Sunday's hosts.
While the Rossoneri come into the weekend unbeaten in four league outings - most notably ending Inter Milan's dominance in the derby - they have not been able to replicate such form against top-class opponents in Europe.
Handed a couple of tough tests in the Champions League, Paulo Fonseca's side were first beaten at home by Liverpool, then edged out by German champions Bayer Leverkusen on Tuesday evening.
Milan have now won only nine of their last 33 games at the elite level, despite making it through to the 2023 semi-finals, and they have plenty of catching up to do if they are to secure a cherished top-eight finish in the new league phase.
Back on the domestic scene, the Rossoneri sat in third spot after six Serie A matches, following last week's 3-0 win over Lecce, as strikes from Alvaro Morata, Theo Hernandez and Christian Pulisic took them top of the goal rankings with 14.
That equals the club's second-best tally at this stage of a season since the turn of the century, proving that defensive doubts may always plague Fonseca's teams, but they often outscore lesser opposition.
Fiorentina Serie A form:
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Fiorentina form (all competitions):
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AC Milan Serie A form:
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L
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W
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AC Milan form (all competitions):
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W
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L
Team News
Aside from retaining Christian Kouame - and shifting the Ivorian forward up front - Raffaele Palladino changed his entire starting XI in midweek; Fiorentina will be back at full strength on Sunday evening.
Palladino switched to a 4-2-3-1 formation for the Derby dell'Arno against Empoli, then continued with that setup versus TNS, but it remains to be seen whether the hosts revert to a more familiar back three to keep Milan at bay.
Defensive trio Luca Ranieri, Pietro Comuzzo and Lucas Martinez Quarta will return after serving UEFA suspensions, which increases Palladino's options, but Croatian centre-back Marin Pongracic is again ruled out by a thigh problem.
Meanwhile, Milan naturally named a strong side to tackle Leverkusen, and few changes are expected for their final fixture before the international break.
Tammy Abraham and Alvaro Morata will either compete for one place or join forces in attack, but Rafael Leao and Christian Pulisic are sure to feature in the final third.
Leao scored his first Milan goal versus Fiorentina back in 2019 and has also found the net in his last three league matches against them; meanwhile, September 'Serie A Player of the Month' nominee Pulisic has scored in each of his last three top-flight appearances.
Marco Sportiello (hand), Ismael Bennacer (ankle) and Alessandro Florenzi (ACL) are all sidelined, but captain Davide Calabria is back in action after overcoming a muscular injury.
Fiorentina possible starting lineup:
De Gea; Martinez Quarta, Comuzzo, Ranieri; Dodo, Cataldi, Bove, Gosens; Colpani, Gudmundsson; Kean
AC Milan possible starting lineup:
Maignan; Emerson, Gabbia, Pavlovic, Hernandez; Fofana, Reijnders; Pulisic, Morata, Leao; Abraham
We say: Fiorentina 1-1 AC Milan
Though Milan's domestic form is on the rise, Fiorentina are rarely beaten on Tuscan turf and should be able to resist their visitors from the second city.