Ben Maher edged his fellow Olympic team gold medallist Nick Skelton in a thrilling finale to claim victory in the inaugural Grand Prix of London.
Maher, 30, took advantage of going out last on his mare Cella in a 10-rider jump-off to finish more than a second faster than Skelton.
"I think I lost it rather than Ben winning," Skelton, 55, said.
On home soil at the Olympic Park in the Global Champions Tour, William Funnell was fourth with Scott Brash fifth.
"My first fast round was the difference between winning and losing. I had nothing to lose," Maher said.
"To get on the podium you had to take every risk and for me it was winning or nothing.
"Normally I don't hear the cheering in the crowds but like the Olympics I did today."
Skelton, who had the penultimate ride, believed he lost the fourth of this season's 15 Grand Prix by not going on the offensive.
"I was very pleased with [ride] Big Star, he's been in good form," he said.
"In a competition like this, to go last is key. In hindsight I would have taken more risk at the last fence. It was a great competition."
Despite the success on Saturday, Michael Whitaker is the highest-placed British competitor in the overall standings after four events in sixth place. Skelton lies 10th with Portugal's Luciana Diniz in first.
At London 2012, Maher, Skelton and Brash were part of the Team GB quartet that captured Britain's first Olympic showjumping gold medal in 60 years after winning a dramatic jump-off against the Netherlands.
Grand Prix of London final standings
1. (GBR) Ben Maher (Cella)
2. (GBR) Nick Skelton (Big Star)
3. (GER) Christian Ahlmann (Taloubet Z)
4. (GBR) William Funnell (Billy Congo)
5. (GBR) Scott Brash (Hello Sanctos)