Chun racked up 7 birdies against one bogey at Columbia Edgewater Country Club to leave the South Korean in position to contend for her first win of 2017.
The 23-year-old, whose two LPGA Tour victories to date have both been major championships, has been four times runner-up in 2017 and is playing this event for the first time.
Chun leads by one over a group of five players including two-time defending champion Brooke Henderson, Cheyenne Woods and the big improver in 2017, Moriya Jutanugarn.
Lexi Thompson, the World No. 2, appeared to be a likely challenger for the lead when she moved within one of Chun through 11 holes of her afternoon round but double bogeyed the 17th after finding a hazard. Thompson then holed a lengthy birdie putt at the last and is just two behind Chun.
World No. 1 So Yeo Ryu disappointed with a round of 74 to be several shots outside the likely cut line and will need a good round tomorrow just to be around for the weekend.
Woods is playing her second season on the LPGA Tour but is struggling to retain her status having missed 12 of 17 cuts this year. At 136th on the current LPGA Tour money list, she has only two more events following this week to turn her fortunes around and regain her LPGA Tour status.
For Henderson, an improved 12th place finish in front of home fans last week in Ottawa appears to have inspired her.
“I think it just suits my eye,” said Henderson when referring to her success at Columbia Edgewater. “I love tree-lined courses and I love when -- it's so green everywhere you look.
"The grass is very lush, and I love that.“
"Just playing here, I have so many incredible memories from two years ago, last year, and I just try to feed of the energy and adrenaline that I felt over the past couple years.”
It was an ordinary day for the Australians with Sarah Jane Smith the best placed after her round of one over par 73.
Katherine Kirk, Su Oh and Sarah Kemp are at 2 over and Karrie Webb is 4 over.
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