The world golf number two Lydia Ko has fallen from 17th into a tie for 32nd after 36 holes of the US Open in Pennsylvania after carding a two over par second round 72.
At the tournament's midway stage, the Aucklander was at two over for the tournament, nine strokes behind the leader, Amy Yang of Korea.
Ko followed a birdie on the 11th at the Lancaster Country Club with a double bogie on the par three 12th and a bogie at 18.
In contrast, Yang hit five birdies and bogie to climb into the lead from a tie for 4th, two strokes clear of Japan's Shiho Oyama (66) and America's Stacy Lewis (67).
The Korean made four successive birdies on the back nine carding a four-under 66 after her opening round 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."
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.
Sunshine on day two replaced overcast skies and a storm that hit Lancaster Country Club on day one, causing a premature halt to the opening round that required 55 players to finish on the following 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.
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.