By Martyn Herman

RIO DE JANEIRO (Reuters) - The capricious nature of road racing revealed itself on Rio's perilous Olympic course again on Sunday as Anna van der Breggen took gold after a horror crash for a teammate and American Mara Abbott was cruelly denied a medal.

After commanding leader Annemiek van Vleuten somersaulted into a ditch after skidding at high speed on the slippery Vista Chinesa descent with less than 20km remaining, Abbott rode solo and was leading with 200 meters of the 137km route left before she was engulfed by three chasers.

Van der Breggen then waited for her moment and powered past Beijing runner-up Emma Johansson of Sweden and Italian Elisa Longo Borghini, who took silver and bronze with Abbott trailing in the dreaded fourth place.

The 26-year-old continues Dutch domination of the Olympic road race since it was introduced in 1984. The cycle-mad nation has won four of the nine editions, but emotions were mixed as Van Vleuten was taken to hospital with back injuries.

A statement from the Dutch Cycling Federation (KNWU) said she was conscious and not in danger.

"She has severe concussion and three small fracture particles in her lumbar spine. She will remain in I.C. for 24 hours," it said on Twitter. "She is conscious and talking."

Van der Breggen expressed concern for her teammate after the finish.

"I won the biggest race of my career so I'm really happy but I'm really hoping Annemiek is okay," she told Reuters.

"It shocked me to see her there. It took me a while to realize that, OK we have to get our heads back on the race and start the chase. I knew we couldn't help her."

Reigning champion Marianne Vos could only finish ninth but played a vital support role to her younger Dutch team mates, sacrificing her own hopes with a series of attacks that took the sting out of several favorites.

World champion Lizzie Armitstead of Britain, dogged by controversy in the build-up after escaping a ban for missed doping tests, was fifth in a compelling race that ignited on the Vista Chinesa climb that caused mayhem in Saturday's men's race.

Van Vleuten and Abbott attacked hard on the 8.9km climb and scaled the summit together before Van Vleuten streaked ahead on the twisty descent as Abbott adopted a cautious line as drizzle added to the treacherous nature of the road that caught out Italian Vincenzo Nibali 24 hours earlier.

Toward the bottom of hill she appeared to lock her wheel and careered headlong over her handlebars into a bank - lying motionless in a twisted heap as Abbott sped by.

While Van Vleuten's hopes ended suddenly, Abbott seeped away and she was tearful at the finish after losing a 28-second lead in the last three kilometers of the race, her pursuers working together to relentlessly reel her in.

"I didn't think about winning it until I got to 200m to go and the second I thought it they passed me," she said before being consoled by 42-year-old team mate Kristin Armstrong.

"It shows you shouldn't count your chickens."

On the crash, she said: "Sometimes in cycling the ones that look really bad aren't as bad so I just hope that she's all right... There's something weird about the surface there."

The first half of the race, along a windswept coast and two circuits of the partly-cobbled Grumari climbs, belonged to 20-year-old Belgian Lotte Kopecky who stayed out in front alone for 70km before being caught.

A series of attacks followed before the day's big climb with Germany's Trixi Worrack taking six riders with her, including Vos, as the final climb loomed.

Vos later faded, but this time she was not Plan A.

"She is a hero of mine and she worked so hard for me today. I'm so proud," Van der Breggen said.

(Reporting by Martyn Herman; Editing by Frank Pingue and Bill Rigby)