Her ailing back a bit better, defending champion Lexi Thompson shot a five-under 66 for a share of the Meijer LPGA Classic lead with In Gee Chun.
Thompson birdied five of the first 13 holes at Blythefield in her bogey-free round to match Chun at 15-under 198.
The 21-year-old American birdied three of the four par-5 holes and is nine under overall on the par 5s.
Thompson won the Honda LPGA Thailand in February for her seventh LPGA Tour title.
The long-hitting Florida player, ranked fourth in the world, also won a Japan LPGA event last month.
The back pain could be a major concern heading into the busy summer schedule and golf's return to the Olympics.
Chun had a bogey-free 65, making four birdies in a five-hole stretch on the back nine.
The US Women's Open champion Chun sat out a month this year because of a back injury after was struck by a suitcase that rival South Korean player Ha Na Jang's father dropped down an escalator at the Singapore Airport.
Sei Young Kim was a stroke back after a 65.
Minjee Lee is the best-placed Australian at six under in a tie for 21st after a disappointing round of 73.
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