Scotland's Mhairi McKay equalled her career best round of 63 in the first round of the Michelob Ultra Open at the Kingsmill Resort in Williamsburg, Virginia.
Still seeking a first win after 10 years on the US-based LPGA Tour, McKay had a faultless round of eight birdies to finish one ahead of former world number one Annika Sorenstam and South Korea's Sun Young Yoo.
"I feel great," said McKay. "I was just so calm and relaxed on the golf course. It was just perfect."
World number one Lorena Ochoa carded a 65, while Scot Janice Moodie was also well placed after a 66.
England's Karen Stupples and Becky Morgan, the only Welsh player in the field, are one shot further back.