Pirazzi, a long-time animator of the Giro and author of many a mountain attack since his debut in 2010, secured his maiden win in Italy's premier stage race after outfoxing four fellow escapees just outside the final kilometre of the 208km stage from Sarnonico to Vittorio Veneto.
The 27-year-old took his rivals by surprise to post an emotional win ahead of Belgian Tim Wellens (Lotto Belisol) and Australian Jay McCarthy (Tinkoff-Saxo).
Thomas De Gendt (Omega Pharma-Quick Step) - the Belgian whose initial attack 30km from the finish fragmented a large 26-man leading group - took fourth place ahead of Frenchman Matteo Montaguti (Ag2R-La Mondiale).
"I knew that between the first five guys it was going to be difficult to arrive together and that I'd have no chance to win from a sprint so I went early," said Pirazzi, winner of the blue jersey for the mountains classification last year.
Like Orica-GreenEdge and FDJ, Pirazzi's Bardiani team now have three wins on the Giro - a superb haul for a second tier domestic outfit.
"We're all young guys at Bardiani and we've had a great Giro so far," said Pirazzi, whose victory followed back-to-back scalps for team-mates Marco Canola and Enrico Battaglin in the second week of the race. "The team is doing a superb job and we can be very proud of ourselves."
An emotional Pirazzi burst into tears after securing the biggest win of his career - and Finnish champion Jussi Veikkanen (FDJ) may have followed suit after celebrating the sprint for what was sixth place - some 38 seconds down - as if he'd just taken the stage win.
On a comparatively relaxed day in the saddle following the controversy and confusion of stage 16 to Val Martello, the Omega Pharma-Quick Step team of former race leader Rigoberto Uran led the peloton over the line a huge 15:36 behind the stage winner Pirazzi.
Nairo Quintana of Movistar finished safely in the pack to retain his 1:41 lead over Colombian compatriot Uran ahead of a decisive three days in the Dolomites.
TWENTY-SIX-MAN GROUP: It took two hours of fast racing before a break finally formed off the front of the peloton following a successful attack by Thomas De Gendt (Omega Pharma-Quick Step) and Josh van Emden (Belkin). The pair were soon joined by Daniel Oss (BMC) before a whole raft of riders sprang out of the peloton in a bit to join the action.
Enrico Gasparotto (Astana), Matteo Montaguti (Ag2R-La Mondiale), Stefano Pirazzi, Nicola Boem and Marco Canola (all Bardiani-CSF), Oscar Gatto (Cannondale), Johan Le Bon and Jussi Viekkanen (both FDJ), Damiano Cunego and Mattio Bono (both Lampre-Merida), Lars Bak and Tim Wellens (both Lotto-Belisol), Igor Anton (Movistar), Serge Pauwels (Omega Pharma-Quick Step), Davide Malacarne (Europcar), Simon Geschke (Giant-Shimano), Alberto Losada and Eduard Vorganov (both Katusha), Philip Deignan (Sky), Evgeni Petrov and Jay McCarthy (both Tinkoff-Saxo) and Fabio Felline (Trek Factory Racing) joined the leading trio with about 120km remaining.
Marco Frapporti, whose Androni-Giocattoli team had missed out on the break, rode solo in pursuit before becoming the twenty-sixth member of the largest break so far on this year's race. Once the peloton sat up, the lead ballooned to well over ten minutes.
Belgian Wellens took maximum points over two Cat.4 lumps before De Gendt made his solo attack just inside the final 30km. Pirazzi was first to respond, catching De Gendt just as the pair crossed the summit of the Cat.4 Muro di Ca'del Poggio climb, which peaked out at a fiendish 18% gradient.
Montaguti, Wellens and McCarthy caught the two leaders with 14km remaining - and despite trailing by just 20 seconds, the other escapees were unable to form a coherent chase. When Pirazzi danced on the pedals with just over a kilometre remaining, the four fellow leaders merely looked at each other - and when they did finally start their sprint to the line, it was too late.
SUMMIT TALKS: The directeur sportifs of all 22 teams met before the stage to discuss what should be done following the race radio debacle on Tuesday that led to many riders believing the descent of the snow-buffeted Stelvio to be neutralised.
Things got rather heated with Patrick Lefevere, the DS of Omega Pharma-Quick Step, calling on race director Mauro Vegni to resign, while there seemed to be unanimous agreement that those riders who had profited from the confusion - the likes of Nairo Quintana, Ryder Hesjedal and Pierre Rolland, the first three riders to cross the finish line - to be docked time.
No decision had been made by the time the remaining 160 riders finally got the stage under way and it seems like this is a debate that will rumble on for quite some time.
BIG WINNER OF THE DAY: Stefano Pirazzi finally secured that elusive Giro stage win - and did so without a high mountain in sight.
BIG LOSERS OF THE DAY: Thomas De Gendt not only sparked the initial attack, it was his move which saw the decisive five-man break come together inside the closing moments. Had the race come to a bunch sprint the Belgian would have surely been the favourite - but he was thwarted by that man Pirazzi.
KEY MOMENT: If it was Pirazzi's counter attack on the Muro di Poggio that ensured De Gendt would not solo his way to the finish line, it was just as importantly Pirazzi's unexpected dig just ahead of the kite which paved the way for his maiden Giro win.
TALKING POINT: Should Quintana be docked 55 seconds for his part in the neutralisation fiasco on the Stelvio? Should he be docked 2:05? Both are figures that were bandied about today - but nothing remains decided.
COMING UP: Before the final two monster mountain stages, we have this intriguing 171km ride back into the Alps which includes the Passo San Pellegrino and the "new" Passo del Redebus ahead of the Cat.1 summit finish at Valsugana. After all the emotions of Tuesday, expect fireworks from the outset between the race's two top Colombians - just as long as it doesn't snow.
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- Thomas De Gendt
- Omega Pharma-Quick Step
- Stefano Pirazzi