Nicola Wilson got Great Britain's bid for an eventing team medal off to a solid start at Greenwich Park.
Wilson and Opposition Buzz, renowned as one of the world's leading cross-country combinations, held their own during the early stages in the dressage phase.
Their score of 51.70 penalties put them in fourth place after 10 starters, and Yorkshire-based Wilson was satisfied with her day's work in front of a bumper crowd.
"I feel that we did the best we possibly could," said Wilson, whose views were echoed by British team manager Yogi Breisner.
"The atmosphere was unreal. The crowd and the cheering was fantastic, and it is just the most beautiful setting.
"Being selected for the Games is obviously a dream come true, something I have dreamt of and worked for since my childhood days. But I have tried not to think about it too much, just stayed in my own little bubble."
Wilson said she was "very, very excited" about the cross-country test awaiting the riders on Monday afternoon.
"It's very hilly and twisting, as we expected, and I am looking forward to preparing him for the next stage of the event. My horse is very experienced and he knows what's happening next," she added.
There was a special moment for young British eventer Emily Llewellyn, who was invited to do a test ride before the main event on Pardon Me II.
"That was seriously cool," Llewellyn said. "It was an amazing opportunity for me, and the arena is incredible."