Pornanong Phatlum and Hye-Jin Choi shot bogey-free rounds of 69 to be Chun's closest rivals at the end of play.
Chun set a major championship record — male or female — when she won the Evian Championship in 2016 at 21 under. She also won the US Women's Open the previous year.
Mixed form for Kiwi men
Also making a strong start to his latest event was Ryan Fox at the BMW International Open in Munich.
Fox fired a flawless opening round 66 that included four birdies and an eagle and left him sitting in a tie for fourth overall. He sits four shots off the lead after China's Haotong Li shot a remarkable 10-under 62.
Meanwhile, at the US Senior Open Championship, Steven Alker - who is line to become a multi-millionaire solely from 2022 earnings this weekend - was tied for 22nd overall after a roller-coaster beginning to the tournament.
A double bogey and a bogey highlighted a slow opening nine on the Saucon Valley Country Club course amidst miserable conditions. However, Alker found a way to claw himself up the leaderboard as steady rain fell, shooting three birdies in the space of four holes on the back nine. A lone bogey then undid his efforts to attain par, but Alker will no doubt feel he prevented further damage with his second-half improvement.
Alker sits five shots behind leaders Jay Haas and Mark Hensby who were a shot ahead of Steve Stricker, Rocco Mediate, Paul Broadhurst and Tim Petrovic. Even with the rain that pounded Saucon Valley in the morning which left greens receptive even when it relented, only 11 players managed to break par.
One of those was fellow Kiwi Michael Campbell who struggled mightily in posting an opening round 79, eight over par.
Also on struggle street was Danny Lee, who was forced to withdraw from the Travelers Championship on the PGA Tour with a hip injury after completing just 14 holes.
- with AP