Lee Bo-mee of South Korea made seven birdies for a 7-under 65 to take a one-stroke lead after Thursday's first round of the Australian Ladies Masters.
U.S. Open champion So Yeon Ryu led early but finished a stroke back at Royal Pines, tied with fellow-South Korean Hee Kyung Seo and Christel Boeljon of the Netherlands.
American teenager Lexi Thompson shot five birdies in an error-free round to sit two strokes off the pace with England's Felicity Johnson and Australian duo Nikki Campbell and Stephanie Na.
The 16-year-old Thompson became the youngest winner in Ladies European Tour history in December when she won the Dubai Ladies Masters. In September she became the youngest LPGA Tour champion with a victory in the Navistar LPGA Classic in Alabama.
Lee was a six-shot runner-up to Karrie Webb in the 2010 Masters.
"Driving good, irons good, putting good. Easy, easy day," she said through an interpreter.
Ryu, who teed off from No. 10, said she felt the weight of expectation after her major win.
"The first few holes I was very nervous as it is my first tournament of the year," she said. "But on the 12th hole I made a nice birdie putt so I could finally relax."
Thompson played with 14-year-old New Zealand amateur Lydia Ko, who became the youngest winner of a professional golf tour event Sunday when she won the women's New South Wales Open by four strokes.
Ko bogeyed two of her last four holes to finish on 2-under 70.
"She's an amazing player. She's so consistent, she barely misses shots," Thompson said of Ko. "She's not scared of anything, you can tell."