The Partenopei sat top of the pile three games ago, but just two points have been collected since then to hand the reigning champions the advantage in the title race.
The majority of the action occurred early on in Florence, where the hosts saw their lead wiped out within 60 seconds.
However, the celebrations were short lived because in the very next attack the away side were level. Argentine striker Gonzalo Higuain had gone four games without a goal prior to this encounter, but he returned to form with a drilled effort from 25 yards out following a mistake from goalscorer Alonso.
A winning goal eluded both sides, although it was not through a lack of effort and attacking play.
For Fiorentina, Cristiano Tello saw his first-half effort cannon to safety off the crossbar, while in the closing stages Higuain had his strike from close range ruled out for offside.