Ruth Edge wowed the judges to go into the lead after yesterday's second day of dressage.
Edge oozed class on board her British Open winner Mayhem III to land six penalties clear of the field on a score of 30.2.
Chasing Edge is young rider Laura Collett.
The ambitious 19-year-old put in a mature and composed performance to score 36.9 on her 2006 Junior European Individual gold medalist Fernhill Sox.
William Fox-Pitt completes the British trio with his Bramham winner Navigator.
New Zealander Andrew Nicholson and America's Bruce Davidson jnr are equal-fourth going into today's cross country phase with a score of 39.
Davidson's father, Bruce snr, won Blenheim in 1994 onboard Squelch and is back in Oxfordshire to watch his son compete.
Piggy French, leader after the first day, dropped to seventh place on What a Performance, with 40.6 points.
Lechlade-based Chris King, who won last year's event on The Secret Weapon, is placed joint-28th going into today's cross country, with 52.5 points.
Elsewhere, Zara Phillips is in 41st place with 51.0 points, while Ginny Howe, from Duns Tew, near Banbury, was first out on Cavort at 9am and finished in 47th place with 56.3 points.
Leading positions (after 2nd dressage): 1 R Edge (Mayhem III) 30.2, 2 L Collett (Fernhill Sox) 36.9, 3 W Fox-Pitt (Navigator) 37.1, 4= A Nicholson (Avebury), B Davidson jnr (Ballynoe Castle) 39.0, 6 A Springer (Arthur) 40.4, 7 P French (What A Performance) 40.6, 8 H Bate (Valdemar) 41.5, 9 W Fox-Pitt (Kaleidoscope) 41.7, 10 B Levett (Hippolyte) 43.3.
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