Carl Hester gave Great Britain a strong start today in their quest for a first Olympic dressage medal.
The 45-year-old, who is riding in his fourth Olympics, scored 77.72% on Uthopia, comfortably the highest score of the opening seven-rider session at Greenwich Park.
"I am very happy - it felt like a very good performance," Hester said.
"I know it's not the highest mark I've had, but I have to take into account it is the Olympics, it is the toughest judges, it is the toughest standard and I am in the first group of competitors.
"I had one obvious mistake, which took me down to under 78%, which would have been my ideal score, but 77% is still great. The horse felt good."
Britain are the reigning European team champions, and their trio of Hester, Laura Bechtolsheimer and Charlotte Dujardin have an outstanding chance of Olympic gold.
A capacity 23,000 crowd watched Hester in dressage action, and he added: "You can't prepare a horse for what we have just done.
"There is nowhere in the world that could prepare you for the atmosphere here. With all that taken into account, I've had a great ride and it is a good start for us."
Bechtolsheimer is the second British rider in action later today, while Dujardin's grand prix test is tomorrow afternoon.
Team scores from the grand prix are combined with those in next Tuesday's grand prix special to decide the team medallists, with Britain expected to face stiff competition from Germany and Holland.