Pippa Funnell's bid for a hat-trick of titles at the Mitsubishi Motors Badminton Horse Trials ended after two falls on the second day.
Lying fourth with Viceroy II after the dressage, Funnell fell towards the end of the cross-country course.
She rushed back from hospital to ride Cornerman, who was lying second but he made the same mistake and she retired after jumping three more fences.
William Fox-Pitt and Tamarillo holds a 2.6-penalty lead overall.
Fox-Pitt, a faller on first ride Moon Man at fence 29, was set to withdraw
Tamarillo because of poor conditions after heavy rain on the steeplechase course.
But selectors' chairman Lucinda Green asked officials to extend the time on that section and he secured the lead over Andrew Nicholson and Lord Killinghurst with a superb round.
"I needed to go clear across country to qualify him for the Olympics," said
Fox-Pitt.
"I am so relieved we went because he never touched a fence and gave me a fantastic ride."
Placings after the cross country: