PORTLAND, August 30, 2013 (AFP) - Pornanong notched the second-lowest round of her career, nabbing 10 birdies with two bogeys on the par-72 Columbia Edgewater Country Club.
She had fired a 63 in the final round of the Mobile Bay LPGA Classic earlier this season.
After recent struggles on the green, the 23-year-old has a new putter in her bag this week, and said the switch was paying dividends.
"Putting was so good," said Pornanong, who needed just 22 putts on Thursday. "This week I changed putter for this week and I'm feeling very good. And I made many birdies today, like 10 birdies today."
Pornanong, who won the unofficial HSBC LPGA Brasil Cup in 2012, is seeking a first official LPGA Tour title.
"I just try my best and I'm feeling very good with the course, the golf course," she said. "I like the course, very nice."
Thompson had five birdies and an eagle on the par-five seventh hole in her seven-under 65.
The 18-year-old is seeking a second title to add to her precocious victory at the 2011 Navistar LPGA Classic. But she said she would try not to get ahead of herself.
"Every round is a learning experience," Thompson said. "Obviously being close to the lead after the first round, you try to take that momentum into every other round after that. But it's golf so you have to take one shot at a time and not get ahead of yourself.
"That's all I'm going to do and try to take this into tomorrow."
Americans Cristie Kerr and Lizette Salas and Germany's Sandra Gal were tied for third on 66, with a group of 14 players three shots off the lead on 67 including world number two Stacy Lewis, 2010 Safeway Classic winner Ai Miyazato, former world number one Tseng Ya-Ni and South Korean veteran Pak Se-Ri.
Another big group on 68 included American Natalie Gulbis, who notched the seventh in-competition hole-in-one of her career at the par-three second hole.
"I'll always take a hole-in-one" said Gulbis, who had four birdies and two bogeys in her round.
Defending champion Mike Miyazato of Japan, who lifted the trophy last year when the event was held at neighboring Pumpkin Ridge, opened with a one-under par 71.
The tournament is without world number one Park In-Bee. The South Korean star, whose six titles this season include a remarkable three major championships, withdrew this week because of illness.
Leading first-round scores on Thursday in the LPGA Safeway Classic (USA unless noted, a-denotes amateur, par-72)
64 - Pornanong Phatlum (THA)
65 - Lexi Thompson
66 - Cristie Kerr, Lizette Salas, Sandra Gal (GER)
67 - Heather Bowie Young, JuliInkster, Lee Il-Hee (KOR), Sarah Jane Smith (AUS), Michelle Wie, Ji Eun-Hee (KOR), Pak Se-Ri, Gerina Piller, Ai Miyazato (JPN), Tseng Ya-ni (TPE), Stacy Lewis, Sydnee Michaels, Austin Ernst
68 - Karlin Beck, Sandra Changkija, Yoo Sun-Young (KOR), Suzann Pettersen (NOR), Natalie Gulbis, Sarah Kemp (AUS), Paz Echeverria (CHI), Dori Carter, Shin Ji-Yai (KOR), Brittany Lang, Amy Yang (KOR), Chie Arimura (JPN), a-Kristina Merkle, Pernilla Lindberg (SWE), Jane Park