So Yeon Ryu took a two-shot lead at the Swinging Skirts LPGA classic after firing a tournament-record 9-under 63.%u200B
Ryu is followed by Haru Nomura and Candie Kung, who are in a tie for second as they carded seven-under-par 65s.
Xi Yu Lin and Christel Boeljon are in a tie for fourth as they recorded five-under-par 67s, while world number one and defending champion Lydia Ko is five strokes off the lead as she is in a tie sixth after registering a four-under-par 68.
As part of the group starting on the 10th, Ryu began her campaign in San Francisco with a duo of birdies at the 11th and 12th before proceeding to make five successive birdies from the 14th to 18th.
Even though the South Korean had a much quieter back nine, she managed to make two more birdies and subsequently broke the previous tournament record, which was held by Brooke Henderson, who carded a seven-under-par 65 in the second round last year.
Nomura, who won the ISPS Handa Women's Australian Open in February, began her round with four birdies on her front nine. She continued her run of good form on her back nine, making a birdie at the second. Even though a bogey followed at the third, the 23-year-old atoned for her mistake with a trio of birdies.
Kung faltered right off the bat as she made consecutive bogeys at the 10th and 11th before finding her groove and ending her front nine with four birdies. She shifted her game to the next level on her back nine as she made two more birdies at the second and third before going on to make a hat-trick of birdies from the fifth to seventh.
Lin made five birdies, two birdies and an eagle at the par-five sixth during her round of 67, while Boeljon made seven birdies and two bogeys.
Meanwhile, Ko, who recently won the Kia Classic and ANA Inspiration, kicked off her round with a bogey at the 11th before turning her fortunes around with four birdies in the space of five holes to round off a successful front nine. Despite faltering with another bogey at the first, the world number one maintained her composure and proceeded to make two birdies to keep herself in contention.
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