Chasing her third victory in four weeks, Pettersen shot a 4-under 68 for an 11-under 133 total after two warm days at Siam Country Club.
'It's another great day on the golf course,' Pettersen said. 'Not too windy. Comfortable temperature. Not so steamy. I'm just trying to play each hole like what they give me. A lot of good shots today and a lot of good putts.'
Canadian Alena Sharp, winless in two full seasons on the tour, matched Pettersen's 68 to hold the second spot. Laura Davies was third, four strokes back at 7 under. She eagled the 18th for a 66, matching the best round of the day on the Pattaya Old Course.
Paula Creamer shot a 66, too, birdieing three of the last five holes to join Joo Mi Kim (67) and Angela Park (70) at 6 under.
Pettersen birdied the par-5 first to get to 8 under, but made four birdies with four bogeys in a nine-hole stretch in persistent light rain that left her back at 8 under as she played the key 13th.
'I have a little work to do when I get back to the range, just adjust a few things. I just have to kind of hang in there,' Pettersen said. 'You have a lot of opportunities out there, so you have to take the mistakes and bounce forward.'
The warm conditions are in contrast to the cold and wind she faced last week in her victory in South Korea. She has four LPGA Tour victories this year, also won in Norway, and has held at least a share of the lead in seven of her last 10 tour rounds.
'I just try to play this week as hard as I can and play smart and we'll see how it comes out,' Pettersen said. 'I feel like I'm playing well. I have a lot of confidence.'
Coming off a second-place finish Sunday in a Japanese tour event, Creamer had seven birdies and a bogey after making three birdies and three bogeys in her opening 72.
'I putted better than today, but yesterday I hit the ball better,' Creamer said.' Tomorrow, I have to hit a lot of greens and make a lot of putts.'
Defending champion Hee-Won Han, making her first start since the birth of her first child this summer, had a 70 to top a group at 5 under.
Annika Sorenstam followed her opening 72 with a 68 to reach the halfway point at 4 under. The 10-time major champion had an early birdie-double bogey-birdie stretch, then birdied Nos. 9 and 10 to get to 2 under and picked up strokes on 13 and 15.
Winless since September 2006, she's making only her 11th tour start in a season marked by a long break because of neck and back injuries.
Catriona Matthew and Kraft Nabisco winner Morgan Pressel rebounded from opening 75s. Matthew was 3 under after a 66, and Pressel's 68 left her at 1 under.
Former Duke star Virada Nirapathpongporn topped the five Thai players in the field, shooting a 71 to reach 1 under. Onnarin Sattayabanphot (71) was even par, and 11-year-old Ariya Jutanugarn shot a 74 to fall to 5 over.
Se Ri Pak and Japanese star Ai Miyazato struggled. Pak was 7 over after a 78, and Miyazato's 80 left her 59th in the 60-player field at 15 over.