LANCASTER, 11 Jul 2015 (AFP) - Yang, who moved to Australia as a teen and now plays out of Orlando, stood on seven-under-par 133 after 36 holes at Lancaster Country Club, recording the second-lowest halfway score in tournament history as she pressed for her first major title.
The 25-year-old South Korean has six top-five finishes in majors but has never won a major crown, her nearest effort coming when she was runner-up of the 2012 US Women's Open.
The only lower first 36 holes in tournament history came in 1994, when Sweden's Helen Alfredsson fired a 132.
Yang birdied the 11th through 14th holes to seize command, hitting 15 greens in regulation while missing only two fairways.
"I'm hitting it very solid throughout the week and it's important to keep the ball on the fairway, especially this big, long rough," Yang said.
"I had good fairway regulation. Just mostly my swing was really good."
Yang, ranked 15th, has never won a US title, taking two wins each on the European and South Korean tours and winning her two LPGA titles in South Korea in 2013 and earlier this year in Thailand.
Yang welcomed the difficult course set-ups typical of US Women's Opens.
"I just enjoy it out here," Yang said. "And I know the golf courses are very tough. And I make sure I'm patient on the course. Just enjoy the tough condition golf courses."
Sharing second on 136 were American Stacy Lewis and Japan's Shiho Oyama with American Marina Alex another stroke back.
Third-ranked Lewis seeks her first victory of the year and will join Yang in Saturday's final pairing. Oyama, who has spent almost her entire career in Japan, fired a 66 to stay on Yang's heels.
Top-ranked Park In-Bee of South Korea was among six players on 138, a pack that also included Australian Karrie Webb, Japan's Rumi Yoshiba, South Korean Chun In-Gee and Americans Morgan Pressel and Jane Park.
Overnight thunderstorms Thursday slowed the greens, Yang said.
"It was a little slower," she said. "But everyone is playing same conditions."
Lewis, who began on the back nine, had three birdies on the front side of the course to stay in the hunt.
"I just started playing golf," Lewis said. "I was thinking a lot those first 10, 11 holes, thinking a lot about my golf swing and just not playing. So I got to just playing golf. I hit the irons today probably the best I've hit them in a really long time, so it was really nice."
In all, 63 players made the cut at 144.
Leading scores on Friday after the second round of the US Women's Open (par-70, USA unless noted):
133 - Amy Yang (KOR) 67-66
136 - Shiho Oyama (JPN) 70-66, Stacy Lewis 69-67
137 - Marina Alex 66-71
138 - Rumi Yishiba (JPN) 70-68, Chun In-Gee (KOR) 68-70, Morgan Pressel 68-70, Park In-Bee (KOR) 68-70, Karrie Webb (AUS) 66-72, Jane Park 66-72
139 - Min Lee (TPE) 71-68
140 - Kim Sei-Young (KOR) 73-67, Ryu So-Yeon (KOR) 72-68, Michelle Wie 72-68, Lizette Salas 71-69, Angela Stanford 71-69, Brittany Lang 70-70, Pernilla Lindberg (SWE) 70-70, Lee Mi-Hyang (KOR) 68-72
141 - Kris Tamulis 72-69, Lala Anai (JPN) 71-70, Megan Khang 71-70, Ryann O'Toole 71-70, Ayako Uehara (JPN) 71-70, Candie Kung (TPE) 71-70, Lee Lopez 71-70, Lee Jung-Min (KOR) 70-71, Baek Q (KOR) 70-71, Ai Suzuki (JPN) 70-71, Azahara Munoz (ESP) 69-72, Choi Na-Yeon (KOR) 67-74