Jessica Korda fired six birdies in a five-under par 67 on Friday to seize a one-stroke lead over unheralded Annie Park in the LPGA Mediheal Championship.
Korda, seeking her second LPGA victory of the season, broke out of a five-way tie atop the leaderboard after the first round as former world number one Lydia Ko, who headlined that leading group, slipped back.
England's Charley Hull carded a four-under par 68 at Lake Merced Golf Club for sole possession of third place on 137.
Ko carded a two-under 70 for 138 and was one stroke in front of Thailand's Moriya Jutanugarn (68) and South Korean Kim Sei-young (70).
Korda notched her fifth career victory at the LPGA Thailand in February as she returned to competition after jaw surgery that sidelined her for months.
On Friday she grabbed three of her six birdies in her first four holes and finished with a total of just 26 putts to edge ahead.
"I knew I was going to get certain bounces that weren't going to be super favorable as the greens got firmer during the afternoon," Korda said. "I just stayed patient and stayed with my process and here we are."
Park, the 2013 NCAA collegiate champion who earned her spot in the field through Monday qualifying, posted the low round of the tournament with a six-under par 66 that included seven birdies.
Park teed off in the morning and led by as many as four. She, too, needed 26 putts, missing just one fairway in a sparkling effort.
"I was hitting it very well today and some putts were dropping," Park said. "It was a good day overall."
Park, is making her first start of 2018, having lost her LPGA card after battling debilitating back trouble.
"It was to a point where I couldn't get out of my bed," she said. "But I learned a lot from it.
"I didn't know how much I missed the sport. I love the sport," added Park, who said the experience had given her "a different perspective towards life and golf."
Ko, who won at Lake Merced in 2014 and 2015, is seeking her first LPGA title since July of 2016.
The New Zealander, who turned 21 this week, closed with her fourth birdie of the round to boost her bid to claim a 15th LPGA title.
"I played really solid today, so it's definitely nice to be able to put myself in better positions," Ko said. "There's still two long days to go, but I'm happy with the way I played the last couple days."
Jessica Korda broke out of a five-way tie atop the leaderboard after the first round with a five-under par 67 to seize a one-stroke lead in the LPGA Mediheal Championship