Walshe shot the lowest round of her LPGA Tour career, a 6-under 65 that included eight birdies over the Highland Meadows Golf Club course.
She has had three rounds of 66 in her career, including one at the Mobile Bay Classic earlier this year.
Lexi Thompson, Paula Creamer and Jessica Shepley were all at 66 while world No. 1 Inbee Park was among those who posted a 67.
Park had her streak of three straight tour victories halted last week, but in two weeks she will attempt to become the first player in men's or women's golf to win four major championships in the same year. That attempt will come at the Women's British Open, to be played at the Old Course in St. Andrews, Scotland.
Hee Young Park, who won last week in a playoff with Angela Stanford, shot a 71 to tie for 44th place. Stanford also had a 71.