Two-time champion Shin Ji-Yai of South Korea shared a one-stroke lead with Japan's Ayako Uehara after the first round in the USLPGA Tour Mizuno Classic tournament on Friday.
The 24-year-old Korean, the winner in 2008 and 2010 who also bagged a major title at the British Open this season, sank five birdies against one bogey for a four-under-par 68.
Uehara carded the same five birdies against one bogey on the 6,506-yard, par-72 Kashikojima Country Club course.
Defending champion Momoko Ueda hit five birdies against two bogeys to trail them on 69, tied for third with fellow Japanese Maiko Wakabayashi, China's Feng Shanshan, South Korea's Choi Na-Yeon, Beatriz Recari of Spain and American Angela Stanford.
Choi, the US Open champion this season, had four birdies against one bogey, while Feng, the LPGA champion, hit five birdies against two bogeys.
American Stacy Lewis, hoping to hold on to her top spot in the Player of the Year standings, hit a 71 for a 19th-place tie with 10 other golfers, including local hope Mika Miyazato and world number one Yani Tseng of Taiwan.