Natasha Baker proved a class apart on her Paralympics debut at Greenwich Park.
The 22-year-old, from Uxbridge in Middlesex, posted comfortably the highest dressage score during Thursday morning's opening session in Grade II of the team competition.
Although Baker is riding only as an individual, her score of 76.095% aboard Cabral proved ideal preparation for her Games campaign.
She will contest individual medals on Saturday and next Monday, when she hopes to repeat her double European gold from last year.
"That was the best experience of my entire life," Baker said. "I woke up this morning and I was fine, but on the bus here I was thinking 'this is actually happening' and reality sunk in a little bit.
"I knew that I had to be calm, cool and collected and get the horse through the test, but he was so good - I am so proud of him. He coped so well with it. We came back with two gold medals from the Europeans in Belgium last year, so to come here with that level of expectation I was a bit nervous.
"He could have gone into the arena and really thought he didn't like it, so I had no inkling what he was going to be like. For him to do that well, I couldn't be more proud of him."
Baker, who contracted transverse myelitis (inflammation of the spine) when she was a child, is one of British dressage's rising stars and despite difficult conditions of driving rain for her test, she delivered a stylish performance as the dressage competition got under way.
"It sets me up really nicely for Saturday," Baker added. "I am just so excited to be here and be part of this amazing event. I'm speechless, really, and that doesn't happy very often!"
Ireland's first team rider Eilish Byrne scored a solid 67.714% with Youri, although she felt the performance was slightly below par, saying: "Youri was a little tense and I struggled to keep him relaxed. Our score was a little lower than usual."