Lucinda Fredericks holds an overnight lead at the Luhmuhlen four-star event in Germany after the dressage phase.
Wiltshire-based Fredericks, who was part of Australia's silver medal-winning team at the Beijing Olympics four years ago, posted a score of 31.80 penalties on Flying Finish.
The 44-year-old, a former Badminton and Burghley winner, held off reigning world and European champion Michael Jung, who spearheaded Germany's challenge.
Jung's anticipated battle for Olympic eventing gold in London this summer against the likes of world number one William Fox-Pitt and New Zealand's Andrew Nicholson is eagerly awaited.
And Jung has put himself in a strong position to push on during Luhmuhlen cross-country phase, lying second with Leopin FST on 32.80.
Nicholson is third aboard Mr Cruise Control, just in front of Australian Andrew Hoy (Rutherglen) and last year's Luhmuhlen winner Andrea Dibowski (FRH Butts Avedon), while Oliver Townend leads the British bid in sixth.
Townend, the 2009 Badminton and Burghley champion, missed out on a place in Great Britain's five-strong team for London.
But he produced a fine showing on his new ride Armada, registering 39.50 penalties, with three other British riders - Charlotte Agnew (Out of Africa Two), Francis Whittington (Easy Target) and Emily Llewellyn (Pardon Me II) - also in the top 11.
Luhmuhlen is the final four-star competition before London, and focus after this weekend will then switch to the Barbury International Horse Trials in a fortnight's time.
Fox-Pitt and his Olympic colleagues Mary King, Zara Phillips, Tina Cook and Piggy French are all expected to give their London-bound horses a final run at the Wiltshire event.
And it is also the final trial for New Zealand's eventing squad before their team to tackle the Greenwich Park terrain is selected.