Ai Miyazato shot a 5-under 67 on Thursday to take a one-stroke lead in the first round of the LPGA Thailand.
On a warm and humid day, the 2010 champion from Japan opened with a birdie and later produced four more at the Pattaya Old Course.
"It was long and hot day for me. But I was calm this morning and could control my shots," Miyazato said. "I made no bogeys but made several birdies. I hit well and putt well. I'm happy to take a lead and keep the score low."
Miyazato said she will be protecting herself from the heat in the coming days.
"I will have use umbrella a lot tomorrow and need some ice," she said.
Five others were one shot behind, including third-ranked Choi Na-yoen. The others are Se Ri Pak, Amy Yang, Anna Nordqvist and Karrie Webb.
Webb made a run on the back nine, sinking four birdie putts to close in on Miyazato.
"The conditions besides the heat were pretty good," Webb said. "We didn't have any breeze. I did think the pins were set pretty tough for first round today."
Numa Kulyanamitta (69) is tied for seventh along with Christel Boeljon, while Stacy Lewis (70) dropped off the top of the leaderboard with a triple-bogey on the last hole.