An emotional Mary King launched her sixth Olympics campaign in masterful fashion to keep Great Britain's eventers on course for medal success at London 2012.
King, at 51 the oldest female Team GB competitor, celebrated the 20th anniversary of her Games debut in Barcelona by thriving under pressure.
She was close to tears after being swept along on a tide of huge Union Jack-waving British support as 20,000 people packed Greenwich Park's main arena.
Imperial Cavalier's dressage test earned a score of 40.90 penalties - his best score at four-star level - that left an elated King in third spot. Had it not been for a poor final flying change, King could easily have taken the overnight lead from German Ingrid Klimke and Butts Abraxxas.
But she provided comfortably the most memorable moments from a home perspective on day one of the equestrian competition, setting up Sunday's action when her British team-mates Zara Phillips, Tina Cook and William Fox-Pitt all make their entry.
Imperial Cavalier can easily be unsettled by large crowds, but King's audience responded to her magnificently when she rode into the arena with a finger pressed to her lips urging silence.
"The atmosphere was absolutely fantastic," King said. "I am not usually tearful, but coming into the arena with the crowd so behind me and being so good at keeping quiet when I came in was amazing. I knew if they all cheered and roared, Imperial Cavalier would have exploded and I would have found it hard to ride the test.
"We made a couple of errors, especially that last flying change when I lost him and he ran a bit to the right which made the change late, but most of his work was really pleasing."
Their score eclipsed Imperial Cavalier's previous best four-star dressage effort of 41.3 at Badminton in 2008, and King added: "I am chuffed to bits. He's got good movement, and the crowds love him."
Nicola Wilson and Opposition Buzz earlier produced a solid test which many observers felt deserved better than the 51.70 score it gained from a three-strong judging panel but with Wilson's strongest discipline, cross-country, to come on Monday, the British team could feel satisfied with their first day's work.