World number one Nelly Korda carded an opening 67 but Sweden's Madelene Sagstrom holds the lead after the first day of the women's golf event at the Tokyo Olympics.
In sweltering conditions at the Kasumigaseki Country Club, Sagstrom hit a five-under-par score of 66, even though she was frustrated by her long game.
The heat was not easy to deal with for the field, with temperatures topping 36 Celsius.
Indeed, Lexi Thompson lost her caddie Jack Fulghum due to heatstroke on the back nine of her round.
"I came into the day having a sore throat... and then dealing with my caddie not being able to finish, it's tough," said Thompson, who carded a 72.
"I mean, I was so worried about him. I'm from Florida and I'm still not used to that kind of heat.
"I just hope that he's okay and he gets the hydration he needs and nutrients to go into the next few days. If not I'll figure something else out. I just want him to be healthy."
Sagstrom, meanwhile, said: "I was striking the ball all right but not great. My short game was my saviour."
Korda, who won the Women's PGA Championship in June, acknowledged how she had also found the conditions difficult.
"I think the mental aspect of it is probably the hardest, just because you have to keep yourself hydrated and you kind of lose it a little out there," said the 23-year-old, whose round included six birdies.
"Like when I was teeing up some balls, I definitely felt a little light-headed, but I kept myself in it."
India's 18-year-old hope Aditi Ashok is level with Korda on four under.
Ko Jin-young had to recover from a poor start but rallied with four birdies in the space of five holes on the back nine to tie with Finland's Matilda Castren and Spain's Carlota Ciganda on three under.
It was a disappointing day for U.S. Open champion Yuka Saso, however, who carded a three-over par 74, a round which included five bogeys.