Alongside riders such as Bauke Mollema (Trek-Segafredo) and George Bennett (Jumbo-Visma), Fortunato was just expected to make up the numbers in the breakaway that eventually formed in the early stages.
However when Jan Tratnik (Bahrain Victorious) attacked part of the way up the famous climb, being tackled from the less steep side, it was Fortunato who jumped out of the group of much more illustrious names to fight his way over to the Slovenian's wheel.
The Eolo-Kometa rider, who has never before won a UCI-registered race, attacked Tratnik with 2.3km to go, on the toughest part of the climb, and managed to hold off the chase to take a memorable win.
Tratnik, finished second with Alessandro Covi (UAE Team Emirates) in third.
"I'm very happy for today," said Fortunato. "The team is the best and in first attacks my teammate Albanese was with me in the breakaway. The breakaway went easy all day but at the start of the Zoncolan I attacked and stayed behind Tratnik. My legs are very good and I'm very very happy.
"I want to thank all of Eolo-Kometa, Luca Spada who is here with me, because without them, I would never have achieved this wonderful result. I came here thinking of getting into breakaways, but I never imagined winning on the Zoncolan."
The breakaway took a fight to establish, but once it went clear it was apparent the 11 riders were in with a very good chance of stealing the win away from the peloton. Fortunato, Tratnik, Covi, Andrii Ponomar (Androni), Remy Rochas (Cofidis), Vincenzo Albanese (Eolo-Kometa Cycling Team), George Bennett, Eduardo Affini (both Jumbo Visma), Nelson Oliveira (Movistar Team), Bauke Mollema and Jacopo Mosca (both Trek Segafredo) moved clear with Affini and Mosca expected to be strong workhorses for their team leaders. The group got out a maximum lead of 8 and a half minutes and reached the foot of the Monte Zoncolan with just over six minutes lead on the peloton.
The final climb was where the fireworks started, Tratnik put in an attack that eased him off the front of the breakaway with 11 kilometres remaining to the finish. Fortunato bided his time as some counterattacks were attempted before putting in his own surge with ten kilometres left.
Fortunato forged his way over to the lone leader Tratnik after a hard chase and the pair joined forces to hold off the remnants of the break and the chasing peloton.
Astana had been a consistent presence at the front of the main bunch, but it was again INEOS Grenadiers who took over on the main sections of the famous climb. Gianni Moscon brought Bernal and the rest of the British team to the front from there it was a case of running down their mountain climbing train as they whittled down the maglia rosa group. Vincenzo Nibali (Trek-Segafredo) was the first major contender to be cut adrift, but many more would follow by the top of the climb.
With 2.3km to go, Fortunato accelerated away from Tratnik but further down the climb the GC contenders struggled along in the wheels of INEOS. Remco Evenepoel (Deceuninck-QuickStep) was the next rider to be dropped, while Davide Formolo (UAE Team Emirates) also went backwards from the rapidly thinning group of contenders for the overall win.
When Daniel Martinez (INEOS Grenadiers) went to the front, the maglia rosa group was down to less than a dozen riders. Simon Yates (Team BikeExchange) had been keeping his powder dry in the early key stages of the race, but he showed his cards on the Zoncolan as he attacked with two kilometres remaining, splitting the group completely as Hugh Carthy (EF Education-Nippo), Damiano Caruso (Bahrain Victorious), Emanuel Buchmann (BORA-hansgrohe) and Alexsandr Vlasov (Astana-Premier Tech) were left scrambling in the wake of the move. Bernal was the only one to cover the acceleration and he continued to follow Yates as the British rider piled on the pressure.
As Fortunato emerged through the mist a the top of the climb to take the stage win, Bernal unleashed his own attack over the top of Yates surging clear. The Colombian finished 11 seconds ahead of Yates and put a lot more time into his other rivals for pink.
Bernal now leads the race overall by a minute and 33 seconds from Yates, who jumped three spots as Vlasov, Carthy and Caruso all fell down the standings.
"I'm happy because finally I've taken time on my closest rivals," said Bernal. "Vlasov was at less than a minute, now Yates is second but he's further back as well.
"I've continued in pink and it didn't rain and wasn't freezing cold, finally, so that was another good thing. I've never known another climb like it, that last part, in particular, was really hard."
The Giro d'Italia continues with Stage 15, a 147-kilometre stage from Gradia to Gorizia that will likely see a sprint finish, though the attackers might have something to say about that with some hills in the finale. Watch the racing action from 2105 AEST on SBS OnDemand, with the SBS VICELAND coverage starting at 2120.