The two-time major winner took advantage of favourable conditions on Thursday at Bethesda, Maryland, to go eight-under in a dazzling start to the third major of the year.

No player has ever been further ahead after 18 holes at a women's major, with Chun's nearest challengers compatriot Hye-jin Choi (69) and Thailand's Pornanong Phatlum at three-under.

Looking to capture another Women's PGA Championship following her 2019 success, Australian Hannah Green (71) was in a group of nine that included defending champion Nelly Korda tied for sixth at one-under.

Green started her round in the rain, opening with a bogey but turned for the back nine at even par. After dropping a shot at the par-four 15th, she followed up with two birdies – draining a 20-foot putt on the 17th.

"The rain stopped, and it was still hard; made a couple of soft bogeys, but super happy to birdie 16, 17 to get me under par," the 25-year-old from Perth said.

RIGHT: In Gee Chun was on fire on day one at Congressional. PHOTO: Elsa/Getty Images.

"Even par is a great score out today ... after the start I'm just glad that I held in there and stayed patient."

Green said while she was stunned by Chun's runaway performance.

"I don't know what golf course she's playing," Green said.

"To hear that she's eight (under), I'm like, 'Wow, that's impressive. That's like shooting 10-under out here.'"

Australia's World No.3 Minjee Lee, who won the U.S. Women’s Open by four shots earlier this month, posted a 73, as did Sarah Kemp, for a share of 30th in a crowded leaderboard.

Of the other Australians, Stephanie Kyriacou was even par, Katherine Kirk was two-over, one shot ahead of Su Oh, while Sarah Jane Smith (78) carded a disappointing opening round of six-over.

Chun's score has only been achieved three previous times in the tournament, the last occasion in 2006, but she's no stranger to going low – winning her second major title at the 2016 Evian Championship with a score of 21-under-par.

The 27-year-old's nine birdies and one bogey set a new best mark for the recently remodelled Blue Course at Congressional, where Rory McIlroy won the U.S. Open by eight shots in 2011.

"The rain stopped, and it was still hard; made a couple of soft bogeys, but super happy to birdie 16, 17 to get me under par." - Hannah Green. 

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-4 15th with four consecutive birdies to make the turn at five-under 31.

A three-time winner on the LPGA Tour and U.S. Women's Open champion in 2015, Chun had her only stumble of the day on the par-4 1st with a bogey.

She then fired four birdies over the next six holes to take the commanding lead.

"The course, after a lot of rain, feels longer," Chun said.

"At the same time, greens were softer. So I think it was just a balance."