British favourites Alistair and Jonny Brownlee will be looking over their shoulders in Tuesday's triathlon after the women's event continued a rich tradition of Olympic surprises.
The brothers from Yorkshire, northern England, who finished first and second in last year's world championship, have such a fearsome reputation they've been warned not to repeat their trick of trying to dead-heat for gold, which is forbidden.
This season younger brother Jonathan, 22, has won races in San Diego and Madrid, before being beaten into second in Kitzbuehel in Australia by world champion Alistair, 24, in June, as he returned from an Achilles injury.
Alistair's seventh victory in his past nine International Triathlon Union (ITU) races, by 49 seconds from his brother, firmly re-established him as the favourite for Olympic gold.
But they will be acutely aware Olympic triathlons are unpredictable affairs, as British hope Helen Jenkins proved in Saturday's women's race -- a gruelling 1.5km swim, 43km bike ride and 10km around London's Hyde Park.
Despite a team designed to position two-time world champion Jenkins among the leaders come the finish, Britain's plan came unstuck when she was unable to kick for home because of a previously unrevealed knee injury.
Instead Switzerland's Nicola Spirig won in Olympic triathlon's first photo finish after a dramatic sprint to the line with Sweden's Lisa Norden.
According to one report, the men's race has been preceded by extensive behind-the-scenes manoeuvring, with the Brownlees forming an informal alliance with strong Slovakian swimmer Richard Varga.
Meanwhile, the Australian team has struck up an understanding with other athletes with the aim of shutting down the British duo.
"The whole international community (is talking) about how to beat the Brownlees," Australian head coach Shaun Stephens told News Ltd.
"We've already seen it in the last couple of races that there are some teams willing to shut them down. Countries know what they are going to do and have to be on the front foot as well."
The ITU has told the Brownlees, who crossed the line together at a British triathlon this year, they'll be disqualified if they try something similar at Hyde Park.
"If they intentionally cross the line together, that is an automatic disqualification for both athletes," said spokeswoman Paula Kim, adding it was a long-standing rule introduced to stop athletes trying to split prize money.
The punishing event has often thrown up unexpected winners at the Olympics. When it debuted in 2000, Australians were supposed to take centre-stage at the Sydney Opera House course.
But in the event, Switzerland's Brigitte McMahon outshone the hosts to win the women's race, and Canadian outsider Simon Whitfield took the men's title.
Four years later, Austria's Kate Allen was the surprise women's winner in Athens, while Hamish Carter and Bevan Docherty took gold and silver for New Zealand in the men's competition.
At Beijing 2008, Germany's Jan Frodeno came from nowhere to pip Whitfield for gold after Alistair Brownlee's early challenge faded, while Emma Snowsill won the women's race for Australia.
Britain's Alistair Brownlee (L) and his brother Jonathan pose on the podium after finishing first and second, respectively, at the Paris triathlon in July 2011. The two will be looking over their shoulders in Tuesday's triathlon after the women's event continued a rich tradition of Olympic surprises.
(L-R) Gold medalist Nicola Spirig of Switzerland, bronze medalist Australia's Erin Densham, Spain's Ainhoa Murua, Britain's Helen Jenkins and silver medalist Lisa Norden of Sweden during the women's triathlon event at the London Olympics on August 4.