Dowsett made it into the day's six-rider breakaway on an unpredictable stage that featured a lot of climbing, with the peloton giving up on the chase at the mid-way point of the day.
Into the final, Israel Start-Up Nation played a tactical masterclass as Dowsett kicked off with the first attack from the break 30km out, attacking once more 17km from the line and pulling clear of his rivals to time trial to a huge solo win.
The stage is Dowsett's second career Giro d'Italia win after his TT victory for Movistar in 2013, and was the first ever Grand Tour victory for Israel Start-Up Nation.
João Almeida (Deceuninck - Quick-Step) holds onto the race lead for another day as the peloton opted not to race.
How it happened
The big talking point before the stage was the abandonments, with five riders not taking to the line in Giovinazzo, most notably Simon Yates (Mitchelton-Scott) who pulled out after testing positive for coronavirus.
Opinions were split amongst the peloton about whether the stage from Giovinazzo to Vieste would swing in the favour of the breakaway or the sprinters.
The 200km stage was split into two sections - a pan-flat opening 90km with the terrain then changing as riders hit the 9.4km-long Monte Sant’Angelo, which crested 106km into the day.
After the descent from the climb, things only got tougher as the course undulated relentlessly over uncategorised climbs, before the fourth-category La Guardiola climb (1.4km at 6.2 per per cent) at 154km.
The course then continued to roll over the next 30km, before the peloton crossed the finish line for the first time, setting up a single 14km lap to the finish line.
It was 25km into the race before six riders pulled out a small gap on the bunch, sparking the day’s breakaway.
Those six were Alex Dowsett and Mattias Brändle (Israel Start-Up Nation), Matthew Holmes (Lotto-Soudal), Salvatore Puccio (Ineos Grenadiers), Joey Rosskopf (CCC Team), and Simone Ravanelli (Androni Giocattoli-Sidermec), with the gap growing out to 10 minutes over the first 100km of racing, as the peloton was content to let the break have a large gap over the Monte Sant’Angelo, Deceuninck - Quick-Step doing much of the pace-setting at the front of the bunch.
Minor drama hit the race 80km from the finish as GC contender Jakob Fuglsang (Astana) suffered a mechanical on the descent from the mountain, leaving him trailing the peloton in a large group a minute behind.
Fuglsang wasn’t able to get in touch over the next 10km, with Trek-Segafredo suddenly deciding it might be an ideal time to chase down the breakaway, but 69km from the line the pace fell away from peloton and the chasers were allowed back in.
The gap between the break and the bunch had tumbled to eight minutes mid-way through the stage, but with 50km left to race it had drawn out more than 10 minutes once more and it became clear the winner would come from the six hopefuls in the breakaway.
Finally the fight for the stage honours kicked off 30km from the finish near the summit of an uncategorised climb.
The other breakaway riders stalled for a moment allowing Dowsett a gap, but Puccio was able to bridge across to the British TT champion and the group came together again after a kilometre.
With 25km left to the line it was Puccio’s turn to attack with Holmes the quickest to follow, that pair pulling out a decent advantage, before being joined once again by Rosskopf, but the gap was just nine seconds to the rest of the breakaway with 20km to go.
The break came together again with Brandle launching a half-hearted attack to set up the full attack from Dowsett, 17km from the line.
Dowsett immediately pulled out a gap as his rivals watched him disappear up the road, using his TT skills to pull out a 33 second gap as the breakaway crossed the finish line to start the final lap and the gap continued to grow.
Over the final climb of the day, with just over 10km to race, Holmes and Puccio put in a huge effort to try and close down the gap, bringing Dowsett back to 26 seconds, but Dowsett was strong enough to extend the gap when he hit the flats, back in his element.
The gap only continued to grow and with 3km to the line Dowsett had 40 seconds, as it became clear he would take the stage alone.
Dowsett looked to be in complete shock as he took his hands off the bars to celebrate his victory, waving one hand in the air and showing how much it meant to him.
Holmes, Puccio and Rosskopf came into the final straight a minute behind Dowsett, with Puccio sprinting to second and Holmes in third to make it a British one and three.
Behind the peloton was in no rush as they came over the final climb of the day, riding in together with no hostilities, 13 minutes behind the winner.
The bunch did hold a half-hearted sprint for seventh, with Michael Matthews (Sunweb) taking the consolation, while points classification leader Arnaud Démare (Groupama-FDJ) and Peter Sagan (Bora-Hansgrohe both sitting out the sprint.
Almeida is still race leader with an unchanged 43-second advantage over Pello Bilbao (Bahrain-McLaren).
The Giro d’Italia continues with a big climbing day on stage nine, 208km from San Salvo to Roccaraso with four categorised climbs and a first category finish.
Giro d'Italia 2020, stage eight: Giovinazzo to Vieste (200km)
1. Alex Dowsett (GBr) Israel Start-Up Nation, in 4-50-09
2. Salvatore Puccio (Ita) Ineos Grenadiers, at 1-15
3. Matthew Holmes (GBr) Lotto-Soudal
4. Joey Rosskopf (USA) CCC Team, all at same time
5. Matthias Brändle (Aut) Israel Start-Up Nation, at 2-10
6. Simone Ravanelli (Ita) Androni Giocattoli-Sidermec, at 2-13
7. Michael Matthews (Aus) Sunweb, at 13-56
8. Fernando Gaviria (Col) UAE Team Emirates
9. Mikkel Bjerg (Den) UAE Team Emirates
10. Andrea Vendrame (Ita) Ag2r La Mondiale, all at same time
General classification after stage eight
1. João Almeida (Por) Deceuninck - Quick-Step, in 29-52-34
2. Pello Bilbao (Esp) Bahrain-McLaren, at 43s
3. Wilco Kelderman (Ned) Sunweb, at 48s
4. Harm Vanhoucke (Bel) Lotto-Soudal, at 59s
5. Vincenzo Nibali (Ita) Trek-Segafredo, at 1-01
6. Domenico Pozzovivo (Ita) NTT Pro Cycling, at 1-05
7. Jakob Fuglsang (Den) Astana, at 1-19
8. Steven Kruijswijk (Ned) Jumbo-Visma, at 1-21
9. Patrick Konrad (Aut) Bora-Hansgrohe, at 1-26
10. Rafał Majka (Pol) Bora-Hansgrohe, at 1-23
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