South Korea’s Yang added a sparkling four-under 66 to her opening 67 for a seven-under-par total of 133.
The 25-year-old Yang, the Open runner-up in 2012 who was tied for first after 54 holes last year, began zeroing in with her irons, taking dead aim at the flags and sinking four successive birdie putts from the 11th hole.
Yang capped her birdie spree by knocking an eight-iron to one foot at the 14th.
“I drove the ball really well,” said Yang, looking for her first major. “I kept my ball on the fairway and close to the pin.”
Tied for second on 137 were world number three Stacy Lewis, who shot 67, and Japanese Tour player Shiho Oyama, who fired a 66 that featured four birdies on her second nine.
“I feel really great and then I’m so excited to play after two more days,” said Oyama. “Chip shot is good, putting is good, everything, I have a confidence.”
Lewis, who finished second to Michelle Wie last year, also got hot on her second nine, making three birdies to move within two of Yang before taking a bogey at the last out of a greenside bunker.
“I hit the irons today probably the best I’ve hit them in a long time,” said two-times major winner Lewis.
American Marina Alex (71) was four strokes off the pace on 137, with six players another shot back — world number one Park In-bee (70), fellow South Korean Chun In-gee (70), Australian Karrie Webb (72), Americans Jane Park (72) and Morgan Pressel (70), and Japan’s Rumi Yoshiba 68).
Sunshine on Friday replaced overcast skies and a storm that hit Lancaster Country Club on Thursday, causing a premature halt to the opening round that required 55 players to finish on Friday morning.
Breezy winds, trickier pins and deceptively soft, slower greens confused the early wave of players when the second round commenced, but the later starters seemed to warm to the task.
Top-ranked Park said it had been hard to get used to slower green speeds since U.S. Open putting surfaces are usually very fast.
“I hit probably every putt short,” she said. “I think that’s what everybody has been doing. It’s hard to think that you have to smash the putts at a U.S. Open.”
Sixty-three players made the cut, which was set at four over par.
Among those missing were former champion Cristie Kerr, who shot 73 for eight-over 148, and Brittany Lincicome (149), a major winner this year.
(Editing by Mark Lamport-Stokes)