Bradley Wiggins made a strong start to his bid to become the first man to win the Tour de France and Olympic gold in the same year.
The 32-year-old last month became the Tour's first British winner and was aiming to carry his form into the 44-kilometre Olympic time-trial at Hampton Court.
Wiggins was the penultimate of the 37 riders to take to the course, assuming his trademark still upper-body position, with legs pumping like pistons, and reached the first time check after 7.3km, in eight minutes 27 seconds.
World champion Tony Martin of Germany, who finished one place ahead of Wiggins in Denmark last September, was quickest in 8mins 22secs, while defending champion Fabian Cancellara of Switzerland was third fastest in 8:28.
Taylor Phinney of the United States was three seconds further behind in fourth, with Chris Froome of Britain fifth.
Luis-Leon Sanchez of Spain suffered a broken chain rolling down the start ramp, losing costly time. He was 39 seconds behind at the first time check.
Any medal for three-time Olympic track champion Wiggins would take his tally to seven in his fourth Games, surpassing rower Sir Steve Redgrave's British record haul.