Double Olympic medallist Pippa Funnell produced a supreme dressage display on Cornerman to move into the lead at the Burghley Horse Trials.
Burghley coverage: Radio Five Live; BBC2, Sunday Grandstand from 1530 BST
Funnell, who won silver and bronze in Athens on Primmore's Pride, picked up just 35.4 penalties.
It leaves her with a 4.8-penalty lead ahead of New Zealand's Andrew Nicholson on Lord Killinghurst.
Afterwards Funnell said of her horse: "His whole standard of work has improved over the past year."
Funnell's Olympic team-mate William Fox-Pitt had to make do with third on Ballincoola.
Fox-Pitt said: "It is the toughest course since 2000, and one of the longest at 12 minutes 40 seconds."
1 Cornerman (P Funnell) 35.4 pen
2 Lord Killinghurst (A Nicholson, Nzl) 40.2 pen
3 Ballincoola (W Fox-Pitt) 42.4 pen
4 Moon Fleet (A Hoy, Aus) 44.4 pen
5 Tangle Man (P Stockton) 44.8 pen
6 Shear H20 (L Law) 46.0 pen
6 Tom Quigley (P Stockton) 46.0 pen