Having already been cautioned just after the restart, Fernandez was shown a red card on 57 minutes for pulling an opponent's shirt.
Nevertheless, the Viola - unbeaten in three previous Serie A matches and with coach Paulo Sousa serving out a one-game touchline ban - immediately rallied to take the lead through Federico Bernardeschi.
Although midfielder Emanuele Giaccherini equalised for the hosts on 63 minutes, Fiorentina held out to secure a hard-earned point at Renato Dall'Ara which keeps them third, but now only two points ahead of Inter Milan who travel to Verona on Sunday morning (local time).
Lazio missed the chance to leapfrog Empoli and Sassuolo in the standings as they were held to a goalless draw by Genoa.
The point leaves Stefano Pioli's men in ninth, while the home side moved up a spot into 15th.
