Oxfordshire's Kitty Boggis won the Network Group International Horse Trials at Gatcombe Park on Kings Cross - leaving newly-crowned world champion Zara Phillips among those trailing in her wake.
Boggis, 24, from Lew, near Bampton, and the ten-year-old were placed second behind Phillips and Red Baron after both the dressage and show jumping in the one-day event.
But Boggis and Kings Cross then produced a tremendous display in the cross country to take victory, posting a clear round and being the only finishers inside the time limit.
Phillips and Red Baron picked up time penalties and slipped back to fifth, with New Zealand's Dan Jocelyn taking second on Savoir Faer and Australian Sam Griffiths and Happy Times third.
Boggis was thrilled to taste victory, and gave much of the credit to her mount, who is owned by Marlborough-based Ali Fox, wife of Jim, who led Great Britain to team gold in the modern pentathlon at the 1976 Olympics in Montreal.
"It was a really good win," said Boggis. "The horse went very well. He has been building up for a big win for a long time.
"He deserved it. He is a very consistent horse and he pulled it out on the day."
Boggis is optimistic the win will help her burgeoning career - last year she competed as an individual at the European Championships at Blenheim - making her the first to ride for Britain at all four levels, having previously competed at pony, junior and young riders.
"It was very important," she added. "It was a good stepping stone in my career and hopefully other people might notice me and it will interest other sponsors.
"It was one of my better wins so far. I didn't win at Badminton, but I had a really good placing there last year when I came seventh which was my real highlight.
"This was a good win though. It was a competitive section. I think there were 104 starters in it and he did very well to get his nose in front."
Now Boggis is looking forward to riding Kings Cross in a three-day event at Weston Park, Shropshire over the weekend of October 5-7.
"Hopefully he should be very competitive there, but at the end of the day a horse is not a machine and anything could happen," she added.
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