Jiyai Shin and Ayako Uehara both shot 4-under 68 on Friday to grab a share of the lead after the opening round of the Mizuno Classic.
Shin, who claimed her second Women's British Open title earlier this year, is a 10-time winner on the LPGA Tour. Uehara, a Japan LPGA player, is playing this tournament for the seventh straight year.
Defending champion Momoko Ueda carded a 3-under 69 and was joined in third place by reigning major champions Na Yeon Choi (U.S. Women's Open) and Shanshan Feng (LPGA Championship), as well as HSBC Women's Champions winner Angela Stanford, Beatriz Recari and Maiko Wakabayashi.
Inbee Park, who has seven top-3 finishes in her last eight starts, headlines a group at minus-2 at Kintetsu Kashikojima Country Club.
Women's world No. 1 Yani Tseng and Stacy Lewis are in a group tied for 19th after shooting 71 on Friday.
Shin played the back nine first on Friday and opened with a pair of pars. She rolled in a birdie effort on the par-4 12th and followed with birdies on 13 and 14.
The 24-year-old Korean parred five in a row around the turn. She grabbed the lead at minus-4 with a birdie on No. 2. Shin also birdied the sixth, but gave that shot back as she tripped to a bogey on No. 9, her final hole.
"Today the weather was so windy, so I played really hard. My strength is accuracy on the fairway and greens, but today I missed a lot of the greens," Shin stated. "But my chipping was good and my putting was good. I made only one bogey on the last hole, but I'm really happy with my score.
Uehara had a birdie and six pars in her first seven holes. She moved to 2- under thanks to a birdie on the par-3 eighth.
Around the turn, Uehara faltered to a bogey at the 10th. She responded with three consecutive birdies from the 11th to get to minus-4. Uehara parred her last five holes to grab a share of the first-round lead.
NOTES: This is the third time Shin has had a piece of the first-round lead in a 54-hole tournament. She won one of the previous two times she was in this position ... The other major champion this season, Sun Young Yoo, is not in the field this week.