(Reuters) - South Korea's Ryu So-yeon fired a tournament-record nine-under-par 63 in the opening group to start and end the day at the top of the leaderboard after the first round of the Swinging Skirts LPGA Classic on Thursday.
Despite being stirred by a wake-up call at around 4 a.m. in San Francisco, Ryu was bright and alert by the time she hit the course and signed for a bogey-free round that gave her a two-shot edge over Taiwan's Candie Kung and Japan's Haru Nomura.
"I hate to wake up early but fortunately (we) had good weather," Ryu told reporters at the Lake Merced Golf Club.
"The temperature was perfect... no wind. Everything was there for me. I'm really happy with my round."
China's Xi Yu Lin and Christel Boeljon of the Netherlands share fourth on five-under, while two-time defending champion and world number one Lydia Ko is among a quartet of players a stroke further back after the New Zealander shot a 68.
Ko is the form player on the LPGA Tour and is looking for her third win of the season, and fourth globally, after she captured the ANA Inspiration at the start of the month to claim her second major tournament victory in a row.
Ryu, meanwhile, is winless since the 2014 Canadian Pacific Women's Open but the 2011 U.S. Women's Open champion has not missed a cut since the Evian Championship in 2014 -- the longest active streak on the LPGA Tour.
Having left her previous instructor to work with double major winner Jordan Spieth's coach Cameron McCormick earlier this year, 25-year-old Ryu appears better prepared to add to her three LPGA titles.
"I thought it would take longer but I think my swing changes are pretty much complete," she said. "Hopefully it will pay off the next three days as well."
Writing by Jahmal Corner in Los Angeles; Editing by John O'Brien