Britain's Emma Pooley could only finish 14th as veteran American Kristin Armstrong won her third successive Olympic Games time trial gold.
Armstrong, who turns 43 on Thursday, stopped the clock on a hilly, 29.9km course just outside Rio in a time of 44min 26sec, which was five seconds faster than runner-up Olga Zabelinskaya (Russia) and 11 seconds ahead of third-placed Anna van der Breggen (Netherlands).
Pooley, 33, came out of retirement predominantly for the Olympic time trial but was never on medal pace, crossing the first time check ninth fastest, the second split 11th quickest and eventually finishing a distant 2min 5sec down on Armstrong.
The race took place in wet and windy conditions, and Pooley admitted the weather had affected her performance.
She said: "I didn't want to just compete at another Olympics; I wanted to get a result, but that's part of sport.
"The weather was a bit different to what we expected. I had to ditch my visor halfway round because it got steamed up. Some people are just better at cornering in the wet, I guess."
Pooley was not the only rider to suffer in the difficult conditions. America's Evelyn Stevens, who is a strong climber and the women's UCI Hour Record holder, could only finish 10th, while reigning world time trial champion Linda Villumsen (New Zealand) had to settle for sixth.
The Netherlands' Ellen van Dijk, meanwhile, might have challenged for silver at least had she not lost concentration and ran off the road on the first of the course's two climbs.
Result
1 Kristin Armstrong (United Stats), 44:26
2 Olga Zableinskaya (Russia), +5sec
3 Anna van der Breggen (Netherlands), +11
4 Ellen van Dijk (Netherlands), +22
5 Elisa Longon Borghini (Italy), +25
6 Linda Villumsen (New Zealand), +28
7 Tara Whitten (Canada), +35
8 Lisa Brennauer (Germany), +56
9 Katrin Garfoot (Australia, +1:09
10 Evelyn Stevens (United States), +1:34
Selected other
14 Emma Pooley (GB), +2min 5sec