Lauren Stephenson holed out from the rough from 137 yards for eagle on the par-4 14th and shot an 8-under 63 for a share of the ShopRite LPGA Classic lead with Mi Hyang Lee.
Stephenson, the 23-year-old former Clemson and Alabama player in her second season on the LPGA Tour, birdied four of the first five holes and finished the morning round on Seaview's Bay Course with eight birdies and two bogeys.
Katherine Kirk is the best-placed Australian at five-under par after carding an opening round 66.
Fellow Australian Hannah Green, using the tournament as a warm-up ahead of her defence of the Women's PGA Championship, is in danger of missing the cut.
Green finished at two-over par with a round of 73 to sit in a tie for 93rd.
Lee, the 27-year-old South Korean player who won the last of her two LPGA Tour titles in 2017, birdied the final two holes in a bogey-free afternoon round.
Defending champion Lexi Thompson, winless since the June event last year, opened with a 67.
The tournament is being contested at 72 holes for only the second time since it began in 1986.
The additional round replaced the pro-am that was cancelled because there are no spectators.
The major Women's PGA Championship is on next week at Aronimink outside Philadelphia.