GUADALAJARA, November 15, 2013 (AFP) - Pornanong birdied the first three holes at Guadalajara Country Club, then birdied seven and nine to make the turn at five-under.
She added two more birdies at 11 and 13 before cooling off slightly with a bogey at the par-four 14th, but her effort was still good enough for a one-shot lead over South Korean Amy Yang, with world number one Park In-Bee, Anna Nordqvist and Ryu So-Yeon a further stroke back on 68.
"Well, today, my putting was very good," said Pornanong, who needed only 25 putts. "I had a complete game today."
Pornanong is looking for a first official US LPGA victory. She tied for second earlier this year at the Mobile Bay LPGA Classic.
Yang also started strong, with four birdies in her first six holes. She added two more birdies and a bogey in her 67.
"I think I did everything so well today, especially putter," said Yang, who notched her first LPGA title at the KEB Hanabank Championship in Korea last month. "I felt really good over the ball every time."
The top two have an imposing trio on their heels including South Korean superstar Park, whose six titles this year include three majors, world number five Ryu of South Korea and former LPGA Championship winner Nordqvist of Sweden.
Park had four birdies without a bogey in her 68, getting off to a good start in her quest to consolidate her claim to Player of the Year honors.
Since winning six titles in the span of 13 events, Park has cooled off this season, and Suzann Pettersen has come on strong. The Norwegian has narrowed the gap in the Player of the Year race, and still has a mathematical chance of seizing the award in the final two events of the year -- this week's 36-woman tournament and the season-ending Titleholders next week.
However, Park needs only a runner-up finish here to be assured of at least a share of the award and said this week it would be nice to "finish it off" before the season finale.
"My ball striking was really good today," Park said. "I hardly missed any shots out there today, I didn't miss any fairways and probably missed only a couple of greens."
Although she knows a victory would make her the first South Korean to win LPGA Player of the Year honors, Park said any tension over the award had subsided once play got under way.
"Today I actually felt a lot more relaxed than when I came in here," she said. "Once the tournament starts, I don't really think about Player of the Year ... I just try to think about the game."
Pettersen was tied for eighth after a first-round 70, bouncing back from an early bogey with three birdies.
First-round scores on Thursday in the US LPGA Lorena Ochoa Invitational (USA unless noted, par 72):
66 - Pornanong Phatlum (THA)
67 - Amy Yang (KOR)
68 - Park In-Bee (KOR), Anna Nordqvist (SWE), Ryu So-Yeon (KOR)
69 - Jenny Shin (KOR), Michelle Wie
70 - I.K. Kim (KOR), Ai Miyazato (JPN), Karine Icher (FRA), Suzann Pettersen (NOR), Lizette Salas
71 - Brittany Lang, Azahara Munoz (ESP), Gerina Piller
72 - Sandra Gal (GER), Jessica Korda, Lexi Thompson, Carlota Ciganda (ESP), Stacy Lewis, Beatriz Recari (ESP)
73 - Caroline Hedwall (SWE), Mo Martin, Morgan Pressel
74 - Chella Choi (KOR), Taylor Collins, Meena Lee (KOR), Paula Creamer, Lee Il-Hee (KOR), Catriona Matthew (SCO)
75 - Jodi Ewart Shadoff (ENG), Margarita Ramos (MEX), Angela Stanford, Alejandra Llaneza (MEX)
76 - Brittany Lincicome
77 - Cristie Kerr