Debutant Laura Collett produced an impressive performance to lead the Badminton Horse Trials after the first day of dressage.
The 21-year-old's routine scored 36.5 points to hand her the overnight lead.
The Wiltshire rider, who has been a junior until this year, told BBC Sport she still has "everything to prove".
But many of the sport's leading lights, such as Mary King and William Fox-Pitt, will have chances to overtake her when the dressage resumes on Saturday.
Collett has racked up impressive results at European junior level, but showed few signs of nerves as she performed for the first time at Badminton, one of the equestrian world's most prestigious three-day events.
She has been compared to her hero, three-time Olympic medallist and equestrian superstar Pippa Funnell, who is also competing here.
The title will be decided after Sunday's cross-country test and Monday's showjumping finale, with Fox-Pitt, Funnell and Oliver Townend among other British contenders for the 2011 crown.
This year's Badminton also doubles as the first major Olympic qualifier for London 2012, with riders out to qualify their horses for the Games and impress British selectors.