Pooley finished second behind Armstrong in Beijing
Olympic silver medallist Emma Pooley missed out on a podium place at the Road Cycling World Championships time trial in Mendrisio, Switzerland.
The Briton finished in 11th, while compatriot and Olympic track silver medallist Wendy Houvenaghel was 23rd.
Olympic champion Kristin Armstrong took gold, with Noemi Cantele in second and Linda Melanie Villumsen in third.
Bradley Wiggins goes in the men's time trial on Thursday and Nicole Cooke in the women's road race on Saturday.
The Isle of Man's Mark Cavendish, winner of six stages in this year's Tour de France, has withdrawn from the men's road race with a respiratory illness.
American Armstrong clocked 35 minutes 26.09 seconds over two laps of the 16.7 mile course, beating Cantele of Italy by 55 seconds and Villumsen of Denmark by 58.
'Perfect day' for Armstong as she wins gold
The 36-year-old Armstrong, who won her first time trial world title in 2006, has said she will retire after these championships.
Defending champion Amber Neben of the United States finished sixth, trailing her team-mate by almost 90 seconds.
French veteran Jeannie Longo-Ciprelli finished 10th, one minute 48 seconds behind. The 50-year-old Longo-Ciprelli is a four-time champion.
Three-time Olympic track champion Wiggins has high hopes after a season that saw him finish fourth overall in the Tour de France.
However, Swiss Olympic time trial champion Fabian Cancellara is favourite to take back the title he won in 2006 and 2007.
Wales' Cooke has admitted she is not in the same form that saw her win world and Olympic road race titles last year.
She leads a seven-woman team, including Pooley and rising star Lizzie Armitstead, in Saturday's event.
"To be truthful I'd say I'm not in the same condition I was last year," Cooke told BBC Radio Wales.
"I had an illness in July that has taken a lot more to recover from than I expected, so I've done my best to try and be at my best.
"I'm really looking forward to it. I've been training on the course in the last week, getting to know the climbs, the descents and how to pace myself.
"It's very tough. We've got two climbs each lap and nine laps to do in three hours. It will make for a very exciting race."
Great Britain will also have high expectations for Peter Kennaugh, who will race for GB-based professional squad Team Sky next season and takes part in Saturday's Under-23 men's road race.
Earlier on Wednesday, Australian Jack Bobridge - who will ride for the Garmin professional team next year - won the men's Under-23 time trial, with Britain's Alex Dowsett seventh, 43.96 seconds behind the winner.
Elite men's road race (nine from): Mark Cavendish, Stephen Cummings, Russ Downing, Chris Froome, Roger Hammond, Dan Lloyd, David Millar, Ian Stannard, Ben Swift, Geraint Thomas
Elite men's time trial: Chris Froome, Bradley Wiggins
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