Reigning U.S. Women's Open champion So under 69 on Friday to climb into the lead after two rounds of the Women's Australian Open, while Hee Kyung Seo used a big round to get into contention.
Ryu sits at six-under-par 140, one shot in front of Seo, whose seven-under 66 on Friday turned her chances around after she began the LPGA season-opener with a 75.
The two are in a familiar spot, competing at the top of the leaderboard. Ryu beat Seo in a three-hole playoff to win the U.S. Women's Open title last July. Seo then went on to win rookie of the year, the same honor Ryu is aiming for in 2012.
"It is still the start of the season," Ryu, who was a member of the Korean LPGA last year, said. "I don't know how I can win the tournament. But the goal is rookie of the year."
First-round co-leaders Stacy Lewis and Sarah Kemp both struggled Friday. Kemp shot a six-over 79 and dropped into a tie for 19th at plus-two, while Lewis persevered through an eventful back nine to post an even-par 73.
During a seven-hole stretch, Lewis posted three bogeys, a triple-bogey, two birdies and an eagle. She reached seven-under, then fell to two-under, and needed the eagle at No. 17 just to make par for the round.
She sits in third at four-under-par 142, tied with Jessica Korda (70), Julieta Granada (72) and Melissa Reid (71).
Yani Tseng, the two-time defending champion, posted a quadruple-bogey at No. 7 and finished with a 76 to drop to even-par overall. Only nine players have been able to stay below par through two rounds.
Seo's round was all the more impressive because she bogeyed two holes, including her first, No. 10. But after that, she ran off three consecutive birdies -- and six in her next eight holes overall -- to make the turn at three-under.
She was cruising until a bogey at the par-four No. 4, but again rebounded immediately. Seo drained consecutive birdies at five and six before adding another at the eighth, ending her ascent. The 66 is the best round of the tournament so far.
"Sometimes when I play this kind of course, I am afraid too much about the course and can't play my game and can't make my own swing," Seo said. "But today I was thinking about routine process and coming on the target and that worked really good."
Ryu didn't have as far to go to get to the top of the leaderboard, having posted a two-under 71 on Thursday.
She didn't get off to the best start Friday, with a bogey on her second hole. But, like Seo, she used a string of birdies to rebound.
A birdie at the par-four 13th got the stroke back, and consecutive birdies from the 16th put Ryu at minus-four heading around the turn. She birdied three of her first six holes on the front nine, but a bogey at the seventh bumped her back to six-under.
Still, that was good enough to hold the lead when many players couldn't stay near the top of the leaderboard.
Lewis' fall was extremely abrupt. After a birdie at No. 13 put her at seven- under, she triple-bogeyed the 14th and bogeyed the next two holes to lose whatever grasp of the lead she had.
Tseng was one shot behind the leaders after the first round, and started to make a charge with birdies at the second and third. But the world's top-ranked player, who won seven times on the LPGA Tour and 12 times worldwide last season, also fell quickly.
In addition to the quadruple-bogey, Tseng bogeyed Nos. five, eight and nine, and made the turn at plus-two. She played a bogey-free back nine, but couldn't make up for the 41-stroke front nine.
American teenage sensation Lexi Thompson, who turned 17 years old on Friday, shot another 74 and is tied for 19th at plus-two. She is the youngest winner in LPGA and Ladies European Tour history, having won the LPGA Classic in September and Dubai Ladies Masters in December.
NOTES: The cut is expected to fall at seven-over-par 153. Among those who missed it was Laura Davies (159), the 2004 and 2009 champion of this event. She was also runner-up to Tseng in 2010...Second-ranked Suzann Pettersen posted a two-under 71 on Friday. She had stumbled to an 80 for her first round...Fourth-ranked Cristie Kerr just made the cut, despite shooting a five- over 78 on Friday.