HOYLAKE, United Kingdom, Sept 13, 2012 (AFP) - Winner of last season's US Women's Open before she even became an LPGA Tour member, 22-year-old Ryu surprised herself with a five-birdie round in the testing, breezy conditions.
Conditions could have been a lot worse, and very strong winds are forecast for the second round on Friday.
Only 11 players broke par, with Taiwan's Yani Tseng, winner for the past two years, and Lydia Ko, the 15-year-old amateur, in the next group on level 72.
Ryu, a winner of the Toledo Classic on the LPGA Tour last month and on the Korean Tour last weekend, arrived in England in relaxed mood.
"I have felt under huge pressure ever since I won the US Open," she admitted.
"After I won a major everyone back home couldn't understand why I didn't win more. But getting my second LPGA title at Toledo was great. Today also feels really good.
"On Tuesday and Wednesday I just thought ‘wow.' The weather was so bad here. But today was much better and I just hit a lot of low fade shots and that really helped. I was only in one bunker and that was also a bonus."
Haeji Kang has never finished higher than fourth on the LPGA Tour but she managed seven birdies, including a great three in a row run from the 14th.
"My iron shots were great and that was the key," she said. "I've played quite a lot of links golf in Australia so that also made it a little easier for me."
Australia's Karrie Webb, a three-time winner, and the 2008 champion from South Korea, Shin Jiyai, were in the group in joint third on 71.
Tseng, who won with a 16 under par total at Carnoustie in Scotland last year, says she relishes tough conditions and the world No.1 remains a massive threat and the championship favourite.
She had four birdies and her only big mistake was a double-bogey at the par four 11th.
"The weather wasn't too bad and I hit a lot of good shots. But I left a lot of birdies out there," she warned.
New Zealand's Ko, who became the youngest winner on the LPGA Tour at last month's Canadian Open, again showed remarkable maturity.
Out first, she opened with a birdie and had four more to confirm that she is ready to challenge for the biggest titles in the women's game. But her feet remain firmly planted.
"Everyone is expecting big things from me," said the teenager.
"But I don't take much interest in what other people say. I'm not going to play well because other people think I should.
"I just play my own game. Today, shooting par was a pretty good start. I could have had a few more birdies but, hopefully, I'm saving them up for tomorrow."
Not that Ko was the top amateur, or the top teenager -- England's 16-year-old Charley Hull snatched the honour with a one under par 71.
Leading first round scores Thursday at the Women's British Open (par 72):
70 - Kang Haeji (KOR), Ryu So-Yeon (KOR)
71 - Stacey Keating (AUS), Karrie Webb (AUS), Lydia Hall (GBR), Charley Hull (GBR), Ai Miyazato (JPN), Shin Jiyai (KOR), Mika Miyazato (JPN), Katie Futcher (USA), Vicky Hurst (USA)
72 - Choi Chella (KOR), Yuki Ichinose (JPN), Amy Hung (TPE), Park Inbee (KOR), Yani Tseng (TPE), Carin Koch (SWE), Lydia Ko (NZL), Katherine Hull (AUS), Becky Morgan (GBR), Seo Hee-Kyung (KOR), Cristie Kerr (USA), Angela Stanford (USA), Holly Clyburn (GBR), Beatriz Recari (ESP), Morgan Pressel (USA), Trish Johnson (GBR)
73 - Paula Creamer (USA), Amy Yang (KOR), Natalie Gulbis (USA), Nicole Castrale (USA), Alexandra Peters (GBR), Candie Kung (TPE), Cindy Lacrosse (USA), Choi Na Yeon (KOR), Brittany Linicome (USA), Dewi Claire Schreefel (NED), Christine Song (USA), Maiko Wakabayashi (JPN), Linda Wessberg (SWE), Han Hee-Won (KOR)