Two-time major winner In Gee Chun equalled the lowest opening round in Women's PGA Championship history and also set the new course record at Congressional Country Club with a brilliant 64.
The 27-year-old posted the early lead at eight under par with a round which has only been achieved three previous times in the tournament, the last occasion in 2006.
Her nine birdies and one bogey set a new best mark for the recently-remodelled Blue Course in Bethesda, where Rory McIlroy won the US Open by eight shots in 2011.
"I feel so good because I always think when my name is going with something, then I feel very honoured and it makes me keep going to play golf," said Chun.
"I'm trying to make the focus on the course for the process, not the result. I'm trying to enjoy to play golf more on the course, more talk with my caddie. I'm happy with the good round today."
Chun is five strokes ahead of Thailand's Pornanong Phatlum and South Korea's Hye-Jin Choi, who are tied for second after shooting 3-under-par 69s. Jennifer Chang and Paula Reto of South Africa are tied for fourth at 2-under.
A group of nine tied for sixth place at 1-under includes defending champion Nelly Korda, Jennifer Kupcho, Hannah Green of Australia, Nasa Hataoka and Ayaka Furue of Japan, Brooke Henderson of Canada and three South Koreans: A Lim Kim, Sei Young Kim and In Kyung Kim.
Sei Young Kim was the 2020 Women's PGA Championship winner, while Green won in 2019 and Henderson won in 2016.
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Chun had a relatively modest start to her round at one under through five holes after starting on the back nine.
She got on a roll starting at the par-four 15th hole with four consecutive birdies to make the turn at five-under 31.
A three-time winner on the LPGA Tour and US Women's Open champion in 2015, Chun had her only stumble of the day on the par-four 10th hole with a bogey. She then fired four more birdies over the next six holes to take a commanding first-round lead.
"I don't know what golf course In Gee is playing!" said Korda.
"She must be playing really well. I would think when I tee it up today, I would be very happy with the score that I shot.
"I think at the beginning part of the day you saw some higher numbers. In Gee is just playing really well, and applause to her. Sometimes it just has to be your week, right? You're going to continue to stick to your game plan and hopefully shoot some good numbers coming into Sunday."
Fifteen players ended the opening day at even par, including Inbee Park of South Korea. The seven-time major winner finished on top of the Women's PGA Championship in 2013, 2014 and 2015.
The purse for this week's third major of the season doubled to $9m, with 99 of the top 100 players on the LPGA Tour money list competing. The winner's share also doubled to $1.35m.
The purse was just $2.25m in 2014.