Ryu, ranked 37th in the world, had a one-stroke lead over nine players, with another seven players two strokes back on 66.
Eight players were tied on 67.
A five-time winner on the Korean LPGA tour, Ryu said a trip home to South Korea to train had led to improvements in multiple facets of her game, particularly her putting, just in time for this 54-hole event.
"I got more confidence in my shot and in the first round I made more birdie chances," she said. "Before this tournament my putting was so bad. But I changed my putting and it's better."
After half a dozen morning starters made it into the clubhouse on six-under on the rain-softened course, Ryu leapfrogged over them with a round that included five birdies on the back nine - four of them in a row from the 14th through the 17th.
Ryu, the LPGA Q-school medalist, has five top-10 finishes this year, including eighth at the US Women's Open at Pebble Beach - the finish on the iconic course providing a massive morale boost.
With that experience under her belt, and her recent work behind her, she's hoping she can hang on to challenge for a victory in a tournament that's traditionally a shoot-out.
"Before I've had three chances for the win but always I'm very awkward and always I miss the putt and make some bad shot.
"But again, I've got a chance."
She'll have to keep the birdies coming on at Pinnacle Country Club, where the group on her heels included major winners Lexi Thompson of the United States, Lydia Ko of New Zealand, Yuka Saso of Japan and Hannah Green of Australia.
They were joined by South Korean Jenny Shin, Germany's Olivia Cowan, Malaysia's Kelly Tan, Denmark's Nicole Broch Estrup and American Christina Kim.
Thompson, among the players who made the trip to Arkansas after competing in the Solheim Cup in Spain last week, is seeking her first LPGA win since 2019.
The American was seven-under through 16 holes but fell back with her only bogey of the day at her penultimate hole, the eighth.
"Overall just a very steady day," Thompson said.
Shin said that was exactly what was needed in a 54-hole event that leaves no room for error.
"This is a birdie-fest golf course," Shin said. "Not making a mistake is absolutely huge."