The 71-year-old Hoketsu was also the oldest competitor at the 2008 games in China. An even greater feat may be that it has been 48 years since his first Olympic appearance: He competed in equestrian show jumping in 1964 in Tokyo.
"The biggest motivation I have to keep competing is that I feel I am improving," said Hoketsu.
Hoketsu scored 68.72 per cent in his Grand Prix dressage test on his 15-year-old chestnut mare, putting him in the middle of the pack of the day's 25 riders and out of medal contention.
"I made two or three mistakes, which I shouldn't have," he said.
Dressage is a sort of equine ballet where the horse performs elaborate gymnastic movements to subtle cues from its rider.
After the Tokyo Games, Hoketsu stepped back from competition and earned a master's degree in economics from Duke University in Durham, North Carolina. Next came a business career in Japan but he still rose at 5 a.m. every day to ride before heading to the office.
After he retired, he renewed his Olympic equestrian aspirations and began training in earnest in Germany.
Hoketsu is the second-oldest Olympian ever: Oscar Swahn won a silver medal in shooting in 1920 at age 72.
Hoketsu could break Swahn's mark if he continued onto the 2016 games in Brazil but doesn't plan to do so — not because of his own energy level but for a lack of horsepower.
"I want to but I can't," said Hoketsu. "It's difficult to find a horse, and mine is now too old."
The second half of the competition's 50 riders will perform their dressage test on Friday. The scores will carry forward to a Grand Prix Special test on Aug. 7 to determine team medals and a freestyle performance on Aug. 9 to decide individual medals.