Sixth-ranked Ryu So-yeon of South Korea fired a bogey-free eight-under par 64 on Thursday to share the first-round lead with American Kelly Shon at the LPGA Meijer Classic.
Ryu, whose lone top-five showing of the season was a share of fourth at Los Angeles, seeks her sixth LPGA title in the Grand Rapids, Michigan, event and her first since winning last June's Northwest Arkansas Championship, which lifted her to world number one.
"This season, I'm not really fully satisfied with it, so hopefully this week it's going to start playing really well and hopefully I can win this tournament," Ryu said.
After starting with back-to-back birdies, Ryu ran off three in a row at the par-three sixth, par-four seventh and par-five eighth holes and added another at the par-five 11th before another set of back-to-back birdies at the par-five 16th at par-four 17th.
It was her low round of the season and her lowest since a 61 during her Arkansas triumph last year.
"I've been working really hard with my coach and we've been really focused on speed control on the greens," Ryu said. "It finally paid off today."
South Korean-born Shon, seeking her first LPGA crown, has missed the cut in six of her 11 prior tour starts this season, with her best result a share of 42nd in Los Angeles.
"I'm doing so many things so well that it's so hard to believe I'm getting what I'm getting," Shon said of her season's struggles. "Tough as it is, I've been still keeping my head down, just working as hard as I could doing the same things, and so yeah, to be honest, I have been waiting for a round like this."
Shon, who began on the back nine, birdied the par-three 12th before taking her lone bogey at 13, which she answered with a birdie at 16.
Shon began the front side, her second nine, with a birdie at the par-five first, added two more at the par-three fourth and par-five fifth, then closed the round with birdies sandwiched around an eagle at the eighth to match Ryu for the lead.
Australia's Su Oh, Sweden's Anna Nordqvist, France's Celine Herbin and American Lizette Salas shared third on 66 with South African Lee-Anne Pace and Germans Caroline Masson and Sophia Popov another stroke adrift.
Canada's Brooke Henderson, the defending champion, was in a pack on 69.
Ryu So-yeon of South Korea waves to the crowd on the 18th green during the first round of the Meijer LPGA Classic, at Blythefield Country Club in Grand Rapids, Michigan, on June 14, 2018