American eventer Karen O'Connor set the early pace at this year's Rolex Kentucky - with British Olympic hopeful James Alliston in sixth place.
O'Connor, who won the competition 13 years ago, guided Veronica to a dressage score of 47.8 penalties, leading midway through an opening day dominated by home-based starters.
New Zealander Andrew Nicholson, a strong contender for Olympic gold in London later this year, was closest to O'Connor with Calico Joe on 48.7.
Alliston, riding Jumbo's Jake, posted 55.0 as the top half of a 57-strong field conducted their dressage tests.
Alliston, an Oxford graduate whose parents live in Gloucestershire, is viewed by some as a contender for Great Britain's five-strong London eventing team.
British performance manager Yogi Breisner is at Kentucky Horse Park watching the season's first four-star event unfold, and Alliston will hope for a strong performance on his top horse Parker on Friday.
World number one William Fox-Pitt, meanwhile, will step up his bid to capture the richest and rarest prize in world eventing on Friday afternoon.
The British star is pursuing the Rolex Grand Slam, awarded if a rider wins consecutive four-star events at Kentucky, Badminton and Burghley in any order. It offers a financial windfall of just under £220,000 and has been achieved on just one previous occasion, by Britain's Pippa Funnell in 2003.
Fox-Pitt landed the Burghley title last September aboard Parklane Hawk, his mount at Kentucky, and he will be among the early starters on day two of dressage.
Shropshire-based Oliver Townend, Badminton and Burghley champion in 2009, will make his Kentucky challenge on recent Burnham Market three-star winner Pepper Anne, while another British Olympic candidate - an in-form Lucy Wiegersma - rides Granntevka Prince.