Cumbria's Ruth Edge took the lead at the 2011 Badminton Horse Trials as the dressage stage concluded on Saturday.
Edge, 31, scored 33.3 penalty points on Two Thyme to ease ahead of Norfolk rider Piggy French with Jakata (36.0).
"I'm chuffed to bits," said Edge. "Two Thyme was a little bit nervous but he went in there and tried his heart out."
Some 120,000 people are expected to attend Badminton's cross-country day on Sunday, before the event finishes with the showjumping stage on Monday.
British stars such as world number one William Fox-Pitt, Mary King and Oliver Townend will need to claw back ground on the leaders during the cross-country.
Badminton's Australian defending champion, Paul Tapner, is placed down in 36th after a disappointing outing on Kilfinnie II, with Fox-Pitt in a similarly lowly 28th and 31st riding Navigator and Cool Mountain respectively.
Townend was visibly frustrated despite a comparatively promising finish of seventh on Ashdale Cruise Master, while King lies 13th with Imperial Cavalier.
Two more British riders, Pippa Funnell and Tina Cook, suffered disappointment as they were forced to withdraw horses early on Saturday.
Funnell's Redesigned came up slightly lame while Cook's Miners Frolic, with whom she won two bronze medals at the Beijing Olympics in 2008, was withdrawn over a swelling which suggested the horse may have been bitten during the night.
Edge, competing just after Saturday's lunch break, snatched the lead with a near-perfect display aboard Two Thyme, a horse aboard whom she has a history of major successes dating back to 2007.
"It's always special here at Badminton," said Edge, who specialises in dressage - competing separately in pure dressage events - and made the most of her expertise in the Saturday sun.
"I know the horse can do it, I know I can do it, but it's just about trying not to let anyone down because it's a lot of effort to get here and a lot of people have helped along the way."
Thirty-year-old French, whose previous best Badminton finish was 31st in 2003, said: "Sunday is another day and we must kick on.
"There's that fine line between it happening like that, and a few little mistakes which put you 10 marks further back.
"I'm just so proud. It's a magic feeling when the horse tries so hard like that for you. It's cool."
Overnight leader Laura Collett, making her Badminton debut at the age of 21, now lies third on Rayef.
This year's Badminton forms a major Olympic qualifying event, though - unlike many sports - the focus is on qualifying horses rather than riders.
The more horses a rider can qualify, the better-placed they will be should an injury or loss of form rule one horse out of contention for the Games.
British selectors, including equestrian performance director Will Connell and eventing performance manager Yogi Breisner, are watching closely at Badminton.
Eventing Olympic selection is currently wide open, with any of 10 or more riders in the frame for the five places on the British team which competes at London 2012.