Heavy morning rain transformed every bump and turn of the 3.85km loop into a hazard, even forcing riders off their bikes to walk some sections of the lush Izu City course near Mt Fuji.

But as rivals slipped and slid around her, Neff burst clear at the 15 minute mark and stayed out of trouble over the 20.55km journey to win in one hour, 15 minutes, 46 seconds.

Teammates Sina Frei (1:16:57) and Linda Indergand (1:17:05) followed her home to fill the podium.

"I knew it was going to be super tough race," said Neff. "This morning we had a complete change in conditions. We had a lot of rain.

"But the Swiss team has invested so much time in skills and technical training and it really paid off."

France's pre-race favourite Loana Lecomte - winner of all four World Cup events before the Olympics - fell off the pace when she dropped a chain while Australian Rebecca McConnell (1:30:29) - third in the last two world championships - finished in 28th place.

She was the last to finish, the 10 others behind her pulled from the race after being lapped.

"(It feels) pretty raw; I don't really know, I just had no energy," McConnell said after her third Olympic campaign.

"To be able to finish felt like it was the biggest mission ever ... the conditions weren't that bad actually.

"Third time (at an Olympics) and still not sort of putting it together is pretty disappointing."

Sixth in Rio's 2016 Games, Neff's gold was the first for Switzerland in the women's event - and the country's first in Tokyo - taking the monkey off the back of the powerhouse cycling nation.

A multiple world champion who also dabbles in road racing, 28-year-old Neff won the Olympic test event at the same course in 2019.

She spent the ensuing Christmas in hospital though with a ruptured spleen, collapsed lung, and broken rib after high speed crash into a pile of logs in North Carolina.

Australian Associated Press