The 22-year-old followed a 63 with a 66 and, on 13 under par, leads by two shots from compatriot Sung Hyun Park (68) and China’s Olympic bronze medallist, Shanshan Feng (67).
Catriona Matthew, the only Scot in the field, had three birdies in a tidy 70, but is 14 shots off the pace on one over par. Charley Hull was the top Briton on one under with Jodi Ewart Shadoff the third to make the cut on two over.
Chun won the US Women’s Open before she joined the LPGA Tour last year and this season she is already well clear in her quest for Rookie of the Year honours.
She also wants a second major. “I’m trying not to think about winning,” she said. “That puts me under too much pressure. I just want to keep playing the same way over the next two rounds.”
Feng, the 2012 LPGA Champion, admitted that the Olympics in Rio had boosted her confidence and she is ready to mount a challenge for her second major and a 17th professional title. “The course played a little longer today, and we had some pretty long holes,” said 27-year-old Feng. “I actually started to feel a little bit tired on the back nine. I think it was because of the walk, and the ground is soft after the rain.
“I made a couple of bogeys in the start of the back nine, but then I talked to my caddie, I was like, let’s finish strong, and then I made two birdies in the last three, so I’m happy about the score
“After the Olympics I went home to China and had a three-week break and only started practising last week. But my putting has been pretty good for the first two days.”
World No 1 Lydia Ko, the defending champion from New Zealand, had a second round 68 and, at nine behind on four under par, is certainly not out of it.