William Fox-Pitt took the individual lead in the eventing dressage and said it felt "special" to be at an Olympics only 10 months after being placed in an induced coma following a fall.
The 47-year-old Briton led with a dressage score of 37.0 penalties after the opening day in Rio.
Fox-Pitt suffered a head trauma in a cross-country fall last October and only returned to action in April.
"I've had an interesting last few months," he said.
"I've been very lucky - I'm lucky to be here. It was very special riding into the arena today. This is my fifth Olympics - they've all been useless! It would be great if this one went well."
Britain lie third behind Germany and Australia in the team competition.
There is a second day of dressage on Sunday, with cross country on Monday and team and individual medals decided by the show jumping on Tuesday.