Mary King ended her 11-year wait for a major four-star title by winning the Rolex Kentucky event in Lexington.
The 49-year-old followed up a third-placed finish with Imperial Cavalier at Badminton last week by dominating America's annual eventing showpiece.
King held the top two places going into the showjumping finale with Kings Temptress and Fernhill Urco.
And both horses jumped clear with Kings Temptress finishing on 47.7 penalties - two ahead of Fernhill - to triumph.
It gave King her first four-star event win since Badminton in 2000 on Star Appeal and landed the Devon-based rider with £80,000 in prize money.
It also gives the five-time Olympian a commanding lead in the FEI Classics Series, which offers a £90,000 top prize for the highest overall finisher in the world's leading five events - Badminton, Kentucky, Luhmuhlen, Burghley and Pau.
"I never dreamt I would be in this position when I left England," said King. "They are two very different horses.
"Kings Temptress is very experienced at this level and was fluent all the way round on cross country.
"Fernhill Urco has no experience at this level, and he was a bit green at the first water jump, but he improved the further he went.
"To be lying first and second after cross country, I had never been in that position before, especially not at this level on two very different horses. I am going to celebrate tonight."
There were also impressive performances from both William Fox-Pitt and Oliver Townend, with both riding horses making their debut in four-star events.
Fox-Pitt was fourth on Neuf des Coeurs, while Townend took sixth aboard ODT Sonas Rovatio after both combinations jumped clear.