Defending champion Lexi Thompson fired a nine-under par 63 on Thursday, matching the course record to grab a two-stroke lead after the first round of the Navistar LPGA Classic.

The 17-year-old American won the event by five strokes last year, becoming the youngest champion in LPGA history at age 16 – a mark broken by 15-year-old New Zealand amateur Lydia Ko at last month’s Canadian Women’s Open.

But Thompson showed no sign of being surpassed at the Robert Trent Jones Golf Trail’s Senator course, firing nine birdies without a bogey to finish the round two strokes ahead of American Lizette Salas and South Korean Hee Young Park.

Thompson had the chance to claim the outright course record, only to miss a birdie putt on the 18th hole.

“I wasn’t thinking about the course record,” she said. “I was just trying to put a good stroke on it like every other putt.”

Thompson, ranked 25th, missed only one green in regulation in matching the course record of 63 set by Japan’s Mika Miyazato and Australian Lindsey Wright in the 2010 event.

Britain’s Karen Stupples, South Korean Mi Hyang Lee and Americans Amanda Blumenhurst, Wendy Ward and world No.2 Stacy Lewis, seeking her third title of the year, shared fourth on 66.

“It was actually a pretty frustrating 66,” Lewis said. “I had a ton of putts go right over the edge and just lip out.”

World No.1 Yani Tseng of Taiwan opened with a 71.