Before the international break intervened, the Bergamaschi had won five of their first seven fixtures and remained unbeaten, while the Viola endured a difficult September but at least ended it with a win.
Match preview
After riding their luck at times to come away from Stadio Olimpico with a valuable victory, Atalanta edged past Roma last time out thanks to Giorgio Scalvini's 35th-minute goal.
The teenager's second Serie A strike took La Dea onto 17 points from their first seven Serie A matches, and in the club's history they have never reached 20 before their ninth game of a top-flight campaign.
Should they manage to do so on Sunday, by beating Fiorentina, it will be a well-drilled defensive line that has taken them to such heights, as Atalanta have flipped expectations on their head by conceding the fewest goals in Italy this season - just three so far.
Previously known as Serie A's great entertainers, Gian Piero Gasperini's latest iteration has gained respect for their resilience at the back, and they have also benefited from last season's meltdown denying them access to European competition this term - extra rest and training time has done them no harm.
In fact, the Nerazzurri briefly led the standings before dropping points in a 1-1 draw with Cremonese which allowed Napoli to surpass them at the summit on goal difference, but they sit second nonetheless.
They may have won each of their last four away games without even conceding a goal, but Atalanta have come out on top just once in their last nine home matches; drawing five times. Therefore, they can take nothing for granted on Sunday.
Not only have their hosts struggled for fluency on home soil this year, but Fiorentina arrive in Bergamo having also won both of the clubs' last two league meetings.
Furthermore, the Viola are unbeaten in eight of their last 10 games at the Gewiss Stadium, winning five times; against no other opponents have they had as much success on the road during the same period.
Vincenzo Italiano's men ended a winless run of five matches when hosting Hellas Verona just before the international break, as goals from Jonathan Ikone and Nicolas Gonzalez gave them only their second league victory of a turbulent campaign to date.
After losing to Udinese and Bologna either side of a draw with Juventus, Fiorentina then crashed to a 3-0 defeat in their most recent Europa Conference League fixture, away to Istanbul Basaksehir.
Having demonstrated great promise in Italiano's first year in charge, they have certainly taken a step back this term, yet remain just inside the top half the table having conceded only six league goals.
Partly due to their troubles at the other end of the pitch, with Dusan Vlahovic yet to be adequately replaced, the Tuscan outfit have lost five of their last six away games in Serie A; scoring only three goals in the process. If they are to secure a first top-flight win on the road since April, then, their forwards simply must do better.
Atalanta BC Serie A form:
D
W
W
W
D
W
Fiorentina Serie A form:
D
D
L
D
L
W
Fiorentina form (all competitions):
L
D
D
L
L
W
Team News
Lengthening Atalanta's absence list, Davide Zappacosta sustained a muscular injury in a practice match during the international period, and is now a major doubt for this week's game.
Defensive pair Jose Luis Palomino and Berat Djimsiti are also unavailable, along with first-choice goalkeeper Juan Musso and star striker Duvan Zapata.
However, Gian Piero Gasperini will at least be able to count on Giorgio Scalvini and Netherlands midfielder Teun Koopmeiners, who have both recovered from minor injuries.
Teenager Rasmus Hojlund or ex-Fiorentina forward Luis Muriel will lead the line up front, most probably supported by Ruslan Malinovskyi. The latter has been directly involved in four goals during his last five league games against the Viola (2 goals, 2 assists).
Meanwhile, Fiorentina's defence has been bolstered by the return of Igor - previously suspended for receiving a late red card against Bologna - and Nikola Milenkovic, who was ruled out of Serbia's Nations League camp due to injury.
Milenkovic's international teammate Luka Jovic has had just 16 touches in the opposition box since moving to Serie A in the summer - fewer than any other outfield player with at least 440 minutes played - but nonetheless competes with Christian Kouame to feature in the visitors' front three.
Long-term absentees Gaetano Castrovilli and Dodo aside, Vincenzo Italiano's squad travel to Lombardy with no major injury concerns.
Atalanta BC possible starting lineup:
Sportiello; Toloi, Demiral, Okoli; Hateboer, De Roon, Koopmeiners, Soppy; Ederson, Malinovskyi; Hojlund
Fiorentina possible starting lineup:
Terracciano; Venuti, Milenkovic, Igor, Biraghi; Bonaventura, Mandragora, Barak; Ikone, Kouame, Sottil