William Fox-Pitt, from Goddington near Bicester, took the early lead in the dressage stage of the Blenheim Petplan International Horse Trials, which started in Woodstock yesterday.
The event began a day earlier than scheduled to accommodate three days of dressage.
Fox-Pitt, the British Open Champion at Gatcombe last month, recorded the best score on Stunning, of the fours riders who competed yesterday.
He is leading the challenge by six Oxfordshire riders against some of the world's best. Fox-Pitt has two rides, Stunning and Tamarillo, in a field of leading riders from 12 nations.
Also taking on the international field will be Georgina Browne from Wantage (riding The Runner Bean II), Graham Law from Chipping Norton (Red Fountain), Owen Moore from Chipping Norton (Farmcote Lad), Tom Robinson from Bicester (Grafham Jupiter and Michael J) and Stefanie Thompson from Hanwell near Banbury (Just Tallulah).
Law excelled in last week's Burghley Horse Trials, where he had the best cross-country score on the way to overall tenth place.
The victorious horse and rider will win 6,000, the rider receiving the Horse and Hound Trophy and the winning owner the Petplan Trophy.
Saturday is cross-country day and the climax will be the crucial show-jumping on Sunday.
Action begins at 9am each day. Accompanied children, 14 years and under, are admitted free, car parking free; adults today and Friday 6, Saturday 14, Sunday 9.
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