Organisers were not able to confirm the report immediately but the high heat and humidity of the Japanese summer is becoming a major headache for Tokyo 2020 planners.
The International Triathlon Union (ITU) had already taken the decision to halve the running section of the race "in the interest of athlete safety" given the "high heat conditions".
The reported incident was the latest in a string of heat-related issues to occur during a series of test events with less than a year to go until the Tokyo 2020 opening ceremony.
On Sunday, about a dozen people fell ill during a rowing test event, with one spectator showing symptoms of heatstroke as the mercury soared above 35 degrees in the Japanese capital.
On the same day, a test event for marathon swimming saw several athletes voice concerns about the water temperature.
Doctors have warned the Games could see deadly medical emergencies in the sweltering Tokyo summer where the mercury can reach 40 degrees Celsius (104 degrees Fahrenheit).
Tokyo 2020 officials have unveiled various measures such as mist sprays, cooling agents and even pots of morning glory flowers to beat the heat during next summer's Olympics.
The suspected heatstroke was not the only controversy at Thursday's triathlon test event as two British athletes were disqualified for crossing the line first holding hands.
Jess Learmonth and Georgia Taylor-Brown fell foul of ITU rules stating that "athletes who finish in a contrived tie situation, where no effort to separate their finish times has been made, will be disqualified".
This handed victory to Bermuda's Flora Duffy as an appeal by the British team was rejected.