Former swimmer Lauren Steadman produced the performance of the day here to upset British teammate Faye McClelland and win the International Triathlon Union (ITU) London World Series Para-triathlon.
Even though the races took place at 8am, well before the men and women's ITU World Series events, a sizeable crowd flocked to the banks of the Serpentine to catch a glimpse of a sport due to make its debut in the Paralympics at Rio 2016.
In the PT4 women's event for athletes with arm impairments the result appeared a foregone conclusion with McClelland having won four world titles during her long unbeaten sequence.
But Steadman, who swam at Beijing 2008 and London 2012 and was part of a British quartet who won 4x100 metres freestyle relay gold at the 2011 European Championshipsn, used her strongest leg to build up a two-and-a-half minute lead.
McClelland produced faster bike and run legs, as well as faster transitions, but the 34-year-old was unable to make up the deficit and eventually finished more than two minutes behind.
Irena Riviera of Italy finished third.
"I just went out to give it my all, try my hardest and hope for the best," Steadman told BBC Sport.
"I beat a four-time world champion, which I never thought would happen this year."
There was plenty more for the early morning home crowd to cheer here as Chris Goodwin and Melissa Reid each triumphed in the respective men and women's PT5 events for visually impaired athletes.
Jane Egan ensured Britain ended the day with four gold medals as she won the PT1 wheelchair class, finishing over seven minutes clear of teammate Karen Darke.
The strong international contingent on show also enjoyed plenty of success, led by a dominant performance by France's Yannick Bourseaux in the PT4 division.
Bourseaux, who has finished either first or second in every international race since 2011, overhauled Spanish leader Jairo Ruiz Lopez to win by just three seconds in a thrilling contest, while Maarten Peters of the Netherlands coming third.
"My goal for this year is the World Championships," said Bourseaux.
"I have to train hard if I want to have the gold."
There were also wins for Italy's Michele Ferrarin in the men's PT2 competition, the Netherlands' Geert Schipper in the PT1, and for Spain's Daniel Molina in the PT3, while there was more French success as Elise Marc, winner of the the women's PT2 event.
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