It was an unlikely but deserved win for van der Hoorn, who attacked clear of his last remaining breakaway companion Simon Pellaud (Androni Giocattoli - Sidermec ) with just under nine kilometres remaining in the race. He then held off the chasing peloton to record a memorable win in Canale.
The win is the biggest of the Dutchman's career, and doubled as a massive moment for his Intermarche-Wanty-Gobert Materiaux team, their first win of the season since being promoted to the World Tour at the end of last year.
"I can't believe it," said van der Hoorn. "I just wanted to go in the break and be aggressive all Giro. It was really difficult to get to the front and win a stage, or get to the finish line with the group."
Van der Hoorn hadn’t won a race since the 2018 Classic Impanis-Van Petegem, 968 days ago. This is his fourth professional victory, the others have been wins on classics and a very similar breakaway win to his Giro success on a flat stage of the WorldTour race, the BinckBank Tour.
"It was a big goal for me to go in the breakaway today," said van der Hoorn. "I had planned that no breakaway would go without me. It wasn’t the plan to finish off like this but I had to take the small chance I had. This first victory of the year means a lot for my new team."
Behind the lone winner, Davide Cimolai (Israel Start-Up Nation) took out the sprint from a reduced bunch for second ahead of Peter Sagan, whose BORA-Hansgrohe team had spent most of the day chasing and dropped many of the other top sprinters on the stage's climbs.
The early breakaway is at times the afterthought of most race reports, particularly when the sprinters are spoiling for a bunch dash finale. However, the 190-kilometre stage from Biella to Canale was set to buck that trend as the move of the day went away with Vincenzo Albanese, who was in the blue jersey having gained King of the Mountains points yesterday, along with his EOLO-Kometa teammate Samuele Rivi.
They were joined by van der Hoorn, Lars van den Berg (Groupama-FDJ), Samuele Zoccarato (Bardiani-CSF-Faizane), Alexis Gougeard (AG2R Citroën) and last year's winner of the intermediate sprint classification Pellaud as the rain-soaked conditions initially saw the peloton more interested in staying dry than pursuing the early move.
After a long struggle to get on, chasing alone for over 10km, the youngest rider in the race, Pellaud's 18-year-old teammate Andrii Ponomar also joined the escapees, forming the final 8-man group that would be the aggressors for the day.
The group pushed out their lead to over six minutes in the early kilometres before BORA-Hansgrohe and Alpecin-Fenix took control of the main bunch to set a pace for their chosen sprinters Peter Sagan and Tim Merlier.
There was a fight out in the break for the mountains points on the three categorised climbs, but Albanese was able to retain his lead in the competition after fighting off some attacks.
Behind, it was a solid pace being set by BORA-hansgrohe on the climbs that caused consternation with the sprinters. Caleb Ewan (Lotto-Soudal) drifted towards the back of the main bunch on the first climb but was not dropped as the group remained intact. There were splits on the descent caused by a crash involving six riders, but the peloton reformed soon after.
BORA-Hansgrohe's pace caused much more damage on the second climb, as sprinters Merlier, Dylan Groenewegen, David Dekker (both Jumbo-Visma) and Ewan were all dropped and taken out of contention for the win.
The German squad continued to push hard on the third and final categorised climb, where they dropped yesterday's runner-up in the sprint Giacomo Nizzolo (Qhubeka-ASSOS).
Out front in the breakaway, Albanese padded his lead in the mountain classification by sprinting for first over the second climb and third on the final categorised climb. The pace-making from BORA-Hansgrohe had significantly reduced their lead over the peloton to less than a minute and thirty seconds with still 36 kilometres to ride.
Pellaud attacked with Zoccarato and van der Hoorn on the uncategorised climb to Guarane 17km from the finish, with Zoccarato being dropped near the top and the front pair forging on together. Behind, there was an attack from the peloton on the short, sharp ascent, with Giulio Ciccone (Trek-Segafredo) and Tony Gallopin (AG2R-Citroen) jumping clear in pursuit of the leaders and a potential stage win.
Pellaud and van der Hoorn were cooperating well however and still had 40 seconds lead with nine kilometres remaining in the race when van der Hoorn attacked.
UAE Team Emirates and Cofidis had seen their sprinters Fernando Gaviria and Elia Viviani survive the climbs and were now the leaders in the peloton as they set a frantic pace to bring back the attackers. Their work did see the gap reduce dramatically, with Ciccone and Gallopin caught 2km from the finish, but Van der Hoorn still had a chance with a lead of 14 seconds in the final kilometre.
Mouth wide, rocking and rolling on the bike, van der Hoorn emerged within sight of the finish line with the peloton still trailing in the distance. Van der Hoorn soaked up the adulation of the fans as he completed a famous win against the odds.
Stage 4 of the Giro d'Italia will be the first test for the climbers with a 4.2 kilometre climb at an average gradient of 9.9 per cent finishing just 2.5 kilometres from the finish in Sestola. The broadcast will start at 1955 AEST on SBS OnDemand, with the coverage beginning on SBS VICELAND at 2125.