Austrian mathematician Anna Kiesenhofer upstaged a heralded Dutch quartet to win a shock gold in the women's Olympic road race on Sunday.

The 30-year-old national time trial champion, a maths graduate of Cambridge and Catalonia universities, held her nerve after a long breakaway that the main peloton never managed to reel in.

Dutch rider Annemiek van Vleuten made up for a horrific crash at the Rio Games in 2016 with silver, while Italian Elisa Longo Borghini bagged bronze for the second consecutive time.

In a dramatic finale to a long day's ride, the peloton reeled in France's Juliette Labous and with under four kilometres to go, the gap to Kiesenhofer was cut to under three minutes.

First, two-time world champion Anna van der Breggen, who was targeting back-to-back golds after winning in Rio, and then Van Vleuten cut loose at the 2km mark in a desperate bid to catch the Austrian, but it was too much too late.

Kiesenhofer shot through the 1km run-in smiling and constantly looking over her shoulder in full knowledge that the gold was hers in a massive upset of the formbook.

Drenched in sweat, she collapsed to the ground after coming through the finish line, screaming in delight, hands on helmet in near-disbelief.

As with the men's road race, tens of thousands of Japanese fans lined the gruelling 137km route between Musashinonomori Park and the Fuji International Speedway that featured 2,692 metres of elevation gain.

There was earlier drama for Van Vleuten, who sustained concussion and a fractured spine in a horrific crash in Rio, after she took a tumble when Denmark's Emma Joergensen fell and veered into her path.

The 2019 world road race champion quickly moved her way back into the peloton and took up the fight, with the women's race featuring a field of 67 riders compared to 130 in the men's road race won by Ecuador's Richard Carapaz on Saturday.

But Kiesenhofer held a five-minute advantage over Van Vleuten for large parts of the race, pushing the pace on a solo descent off Kagosaka Pass 40km from the finish to drop early breakaway partners, Israel's Omer Shapira and Polish rider Anna Plitcha.

And with 24km to go, Van Vleuten was reeled back in by a 14-strong chasing group, stretched out as riders sought to surge.

Hitting speeds of 60km/h (40 miles per hour) on the Fuji race track, Labous was first to push out before the Dutch quartet manoeuvred into an attacking position, albeit in the knowledge that they could nothing about Kiesenhofer's surprise victory in the shadow of Mount Fuji.