Paul Tapner and Kilronan lead the Mitsubishi Motors Badminton Horse Trials after the first day's dressage.
In wet conditions, the Wiltshire-based Australian and the elegant grey performed a faultless and flowing test to finish on a score of 36.
Tapner's score left him 0.2 of a penalty point ahead of early leader and top Brit Francis Whittington on Easy Target.
"I have a tendency to over-think my tests sometimes, but today was the most relaxed I've ever been – I think partly because I know that come Saturday the competition will probably be turned on its head," said Tapner, who won the Badminton title in 2010.
"Kilronan had been turning in tests in the 30s at three-star level, but for a couple of seasons I'd been scratching my head about why we couldn't replicate this at four-star. I spent time analysing every step and working out where we were losing marks, and I was really pleased with my test today."
Another former champion, Lucinda Fredericks, scored 39 on Flying Finish for third spot.
The 2007 winner said: "I've walked the [cross-country] course twice now and it's most definitely a true four-star test – and the ground conditions are going to make it tougher.
"The last time I rode in the real wet at Badminton was probably in 1992 aboard Arctic Goose. Flying Finish has had a good lead-up to Badminton. His last run was at Bicton, which, luckily, was in the wet."
Woolard's Dani Evans will make her debut at the world's premier three-day event when she takes Raphael II into the dressage arena on Friday at 10.08am.
