After breaking the course record at the LPGA's Swinging Skirts Classic, So Yeon Ryu has credited her caddy with the result.
The South Korean rolled in nine birdies for a flawless nine under 63 at Lake Merced, which included a scintillating back nine, finishing with five gains in a row.
Ryu revealed that caddy Tom Watson recommended she used a practice stroke before each putt to help her visualise the stroke.
"I never had any practice stroke before I putted, but Tom recommended me just to have a practice swing, practice stroke behind the putt," the 25-year-old told lpga.com.
"I think it's really helped me to get better visualisation. Yeah, so I think that was the biggest change of my putting. One thing I can say is I work really hard and my whole team work really hard. Hopefully, it's going to pay off next three days as well."
Ryu added that her early tee-off time allowed to make the most of the benign conditions in California.
"Well, I was kind of lucky to tee it off really early," added Ryu.
"I had a really fresh green, no wind. Also, San Francisco is really cold in the morning, but this morning it was just perfect temperature. Then I think today everything was just great. My tee shot was great. I only missed one green today. I shot 17 greens. Putting was really good."
Ryu will head into Friday's second round with a two-shot lead over Haru Nomura and Candie Kung.
Scotland’s Connor Syme and Englishman Matthew Jordan were also part of the successful quintet.
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The event was last staged in 2002.
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