Britain's Oliver Townend, ranked world number one, rode his stallion Ballaghmor Class to an early lead on Friday in the dressage round of the Olympic equestrian eventing competition.
China's Alex Hua Tian came second and Germany's Julia Krajewski third, with team scores led by Britain, Sweden and Japan at the Baji Koen Equestrian Park, the venue for the equestrian events the last time the Olympics took place in Tokyo in 1964.
The rankings could change, however, with further athletes competing in dressage on Saturday including Germany's Michael Jung, who won individual eventing gold in London and Rio.
The cross-country and show jumping tests in the eventing competition are scheduled for Sunday and Monday.
Starting on Saturday, Australia's Andrew Hoy, 62, could make Olympic history if he wins another gold medal 29 years after first claiming the top spot on the podium in Barcelona in 1992. Hoy took part in his first Olympics in 1984, and his eighth Games participation in Tokyo is an Australian record.
Jonelle Price from New Zealand, world number seven, claimed 13th spot on Friday, and will be closely watching how her husband, team mate and world number two Tim performs on Saturday.
A rule change meant the teams can no longer drop their weakest scores and Price said that might have contributed to her "playing a bit of a safer game".
Equestrian sports do not differentiate on the basis of gender and it is not unusual for athletes beyond the age of 50 win medals.
(Reporting by Shadia Nasralla; Editing by Kenneth Maxwell and Ed Osmond)
