Bouwman managed to just force his way ahead into the final corner of the 178 kilometre stage, with the summit finish coming down to an unlikely sprint to the line.
The race organisers had clearly thought the possibility unrealistic as well, with a sharp corner with a fast entry 50 metres out from the finish line causing havoc as the breakaway battled for the stage win.
Bouwman's dive over the top of Mauro Schmid (QuickStep Alpha Vinyl) forced the Swiss rider to brake and maintain a straight line, in turn forcing Andrea Vendrame (AG2R-Citroen) and Attila Valter (Groupama-FDJ) wide and into a run-off meant for race convoy vehicles.
“I didn’t make a mistake. I deserved this victory,” Bouwman said after the stage.
"I knew there was the left hand corner at 100 metres to go. The only chance for victory was to go first into the corner. I started my sprint from fourth from the group and went over them.
"But I didn't know the corner was that tight and I went into it with a lot of speed, around 40kph. I had to brake and so did the guys behind me. Nobody crashed. I don't think I made a mistake."
While riders remonstrated in frustration as they crossed the finish line behind, it was mission complete for Bouwman, who also wrapped up the mountains jersey with the performance. He will now only need to complete the final two day's racing to stand on the podium in Verona.
“The goal of today was to have the blue jersey at the end of the day," said Bouwman. "Coming with a stage victory - I can’t believe it.”
The early breakaway for the 178-kilometre stage was a hotly contested one, with the very real possibility of a winner emerging from the early move.
It didn't take as long as some of the protracted affairs from earlier in the Italian Grand Tour for a move to go free, and well before the major climbs of the day a break was established with Davide Ballerini (QuickStep-AlphaVinyl), Andrea Vendrame (AG2R-Citroen), Tobias Bayer (Alpecin-Fenix), Clement Davy and Attila Valter (both Groupama-FDJ), Magnus Cort (EF Education-EasyPost), Koen Bouwman, Edoardo Affini (both Jumbo-Visma), Mauro Schmid (QuickStep Alpha Vinyl), Edward Theuns (Trek-Segafredo) Fernando Gaviria (UAE Team Emirates) and Alessandro Tonelli (Bardiani CSF Faizané).
Bouwman, Valter and Schmid emerged the strongest on the hardest climb of the day, a mid-stage excursion into Slovenia taking the riders up the Kolovrat as BORA-hansgrohe set the tempo behind. They were joined by Vendrame and Tonelli, forming the group of five that would contest the finish.
BORA-hansgrohe decided to stop working at the head of the peloton which saw the race from the main bunch slow significantly heading into the final climb and the gap blew out to eight and a half minutes to the breakaway.
That led to odd scenes on the final climb to Santaurio di Castemonte as the five riders played it very cagey on the final climb, watching each other constantly for the threat of attack. There were a few surges, with Bouwman the main protagonist, but the battle for the stage win would come down to the flashpoint of a very tight final corner with 60 metres to the finish line.
Schmid had the inside line on the corner, but Bouwman came with more pace on the outside and hit the apex of the corner, forcing Schmid to brake to avoid crashing, which saw him lock up his rear wheel and continue straight, forcing Vendrame and Valter to do the same.
That left Bouwman with a relatively straightforward run to the line to claim the win, with a frustrated gesture from Schmid behind as he crossed the line for second, Tonelli third.
The main GC battle saw some brief, intense skirmishing on the final climb between race leader Richard Carapaz (INEOS Grenadiers), second-placed Jai Hindley (BORA-hansgrohe) and third overall Mikel Landa (Bahrain Victorious).
Carapaz launched an attack as teammate Pavel Sivakov finished his final turn of pace but Hindley and Landa were awake to the threat and they responded immediately, though all other riders in the general classification fight were left behind.
Hindley would attack on a few separate occasions as well, and Landa tried a couple of moves, but each attack was marked out by their main rivals and the trio came into the finish in their general classification order, with Carapaz retaining his leader's jersey, a step closer to securing it in Verona.
In the end, the biggest GC development of the day was that his teammate and key mountains support rider Richie Porte had to abandon during the stage with gastroenteritis. This brings to an end the career of Australia's most successful Grand Tour racer since Cadel Evans with the Tasmanian stating that the Giro would be his last three-week race.
The Giro d'Italia now will be decided this weekend, with a mammoth Dolomites stage, ending on the summit finish of the Passo Fedaia (Marmolada) and Sunday’s final time trial to decide who wins the coveted maglia rosa.
The Giro d'Italia continues tonight with a decisive stage in the overall battle for the pink jersey, with a monstrous day of climbing set to see the top climbers in the race battle it out. Watch the full stage from 8.05pm AEST on SBS On Demand with the SBS coverage starting at 10.30pm AEST. WA viewers can watch from 8.30pm AWST.
