She posted an 11-under 205 total while Canadian Brooke Henderson shot 70 to be alone in second place on eight-under.
South Korean Park Sung-hyun, who shared the halfway lead with Ryu and Henderson, carded 71 to slip four strokes behind.
"After the front nine I couldn't actually expect to be leading after the round," Ryu said after making four back-nine birdies to surge pass a spluttering Henderson.
Ryu gave herself a top grade.
"I think for long game, I can give myself an A, and then about the putting, also I can give myself an A.
"Everything combined really well, so hopefully tomorrow is going to be even A+."
As well as she played, Ryu also got an assist at the par-five seventh, where her second shot landed in a woman's food container, probably preventing the ball from going into a water hazard.
"For sure that one of my luckiest moments out there," she said. "I could have made double-bogey instead of (par), but the little bag was there, so it saved me."
Second-placed Henderson, winner of the event two years ago before finishing runner-up last year, rued slipping from two shots in front at the turn to three behind at day's end.
"I struggled a little bit with my putter, which is disappointing," she said.
"(I will try to) get better in my head and just go out tomorrow and made a lot of good shots and see if I can make some birdies, and hopefully they roll in."
A victory by Ryu would give her three legs of the grand slam, after the 2011 U.S. Women's Open and the 2017 ANA Inspiration.
"After I won the ANA Inspiration, I started to dream about becoming a grand slammer," said Ryu, who turned 28 on Friday.
"If I win tomorrow, it's going to be even better week. You know, it's going to be my best birthday week ever."
(Reporting by Andrew Both in Cary, North Carolina, editing by Ed Osmond)
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