Chun took a two-shot advantage Friday at the halfway stage of the Evian Championship, the fifth and final major tournament of the women's season.
The 22-year-old LPGA Tour rookie equaled the best score of the second round, shooting a five-under-par 66 to move clear of her fellow first-round leader Sung-Hyun Park and Olympic bronze medalist
Shanshan Feng of China.
Chun won last year's US Women's Open after making up a four-shot deficit, though she missed the cut in July's title defense.
She has yet to win this year, but leads the Rolex rookie standings
by a considerable margin after posting six top-three finishes and another three top-10 placings.
"I always try not to think about winning because it gives me more pressure, so I just try to focus on my game," Chun told the LPGA website in France.
But she added: "I like pressure. I always play really good under pressure."
Feng joined playing partner Park on 11 under after 36 holes, outscoring her 67 to 68 despite two late bogeys, as they held the clubhouse lead before Chun completed her round.
"She really surprised me because she looks really skinny, but she hits it really long," Feng said of Park, a non-LPGA member who tied for third at the US Open after leading at the halfway stage.
"She's a very, very good player. She's long but accurate. That's really hard to do. And her putting is really good, also."
South Korea has a third title challenger in double figures after 2011 US Open champion So-Yeon Ryu also carded 66 to be in fourth place on 10 under.
Ryu, who hasn't dropped a shot in two rounds, was one clear of Angela Stanford -- whose fellow American Brittany Lincicombe matched Chun and Ryu to be tied for sixth with South Korea's Eun-Hi Jee (68) on eight under.
World No. 1 and defending champion Lydia Ko
of New Zealand was tied for 13th after a 68, while second-ranked Thai Ariya Jutanugarn was equal 27th following a 67.
US Open champion Brittany Lang
was one of the big names to miss the cut, along with Laura Davies, Juli Inkster, Christina Kim and Karrie Webb.