The Olympic women's marathon will start at 12am Kenyan time (6am) in Sapporo on Saturday, one hour earlier than initially planned due to the "weather", a Tokyo 2020 official said on Friday.
There is no change at this stage to the men's start time of 7 a.m. on Sunday, the official added.
The International Olympic Committee decided in 2019 to move the marathon to the northern city of Sapporo to escape the worst of Tokyo's summer heat.
But Sapporo was just as hot as Tokyo on Friday with the mercury hitting a high of 35 degrees Celsius (95 degrees Fahrenheit).