BANGKOK, Feb 21, 2013 (AFP) - Third-ranked Lewis blistered to the top of the leaderboard with a nine-under 63, lit up by an eagle and eight birdies -- including a hat-trick on holes 5, 6 and 7. She dropped just one shot on her way back to the clubhouse.
The American, who is using a new driver this year, tied her personal best score and the tournament record at the Pattaya Siam Country Club.
She leads second-placed Karine Icher of France, who notched six birdies without dropping a shot in her six-under 66, with Scottish veteran Catriona Matthew leading a clutch of players a shot back at five-under.
But the round belonged to Lewis, who said a "great day" was set in motion by her eagle on the third hole.
"I don't know, that shot just really gave me the confidence to really start swinging at it," the elated American said.
"I think I missed two greens all day. I just had a ton of birdie putts and I even left a few out there. The fact that it could be lower -- I'm not complaining -- but it was just a great day."
There could not have been a greater contrast in fortunes with world number one Tseng, who blundered through the round to finish with a three-over 75, 12 shots off the leader.
The 24-year-old is trying for her first victory in nearly a year -- a barren spell which has seen her 106-week hold on the top ranking slip to a 1.29 points lead over South Korea's Choi Na-Yeon.
She started the day badly, dropping a shot on the par-five first and struggling throughout her round, with a further four bogeys only slightly offset by a pair of late birdies.
It was a far cry from last year's dominance of the Thai event, her second straight win of the competition, which looked set to propel Tseng to another all-conquering season on the tour.
But the victories dried up in March, raising pressure on the young star.
Tseng got off to an encouraging start to the 2013 season last week by finishing second at the Women's Australian Open, behind winner Shin Ji-Yai but ahead of teenage prodigy Lydia Ko.
New Zealand amateur Ko, 15, who became the tour's youngest winner last year, may well turn the tables in Pattaya after posting a solid three-under 69, with two late bogeys on 16 and 17 marring a solid round.
Leading first-round scores at the Honda LPGA Thailand in Pattaya on Thursday (par 72):
63 -- Stacy Lewis (USA)
66 -- Karine Icher (FRA)
67 -- Catriona Matthew (SCO), Amy Yang (KOR), Inbee Park (KOR), Gerina Piller (USA)
68 -- Beatriz Recari (ESP), So-Yeon Ryu (KOR), Lizette Salas (USA),
69 -- Ai Miyazato (JPN), Lydia Ko (NZ), Karrie Webb (AUS), Ilhee Lee (KOR), Mika Miyazato (JPN), Ariya Jutanugarin (THA), Angela Stanford (USA), Mi Jung Hur (KOR), Caroline Hedwall (SWE), Se Ri Pak (KOR)
70 -- Shin Ji-Yai (KOR), I.K Kim (KOR), Eun-Hee Ji (KOR), Azahara Munoz (ESP), Ayako Uehara (JPN), Brittany Lang (USA)
Selected scores:
74 -- Michelle Wie (USA)
75 -- Tseng Yani (TPE)