As most of the nine-strong Australian contingent made a disappointing start, Thitikul claimed a two-shot lead over Haeran Ryu and Shiho Kuwaki at golf's final major of the year.
One of only two players to have been No.1 in the women's world rankings without ever having won a major, Thitikul made three straight birdies early in her afternoon round and kept bogeys off her card the rest of the way.
The 23-year-old Thai has been overlooked during the LPGA major season dominated by Ryu and Nelly Korda, who have split the four majors. Korda, who replaced Thitikul at No.1 in April, won the first two and Ryu the next two.
Ryu put herself in good position to make it three in a row, a feat not achieved since Inbee Park in 2013, with two birdies on her last three holes for a 66. The South Korean won the Women's PGA at Hazeltine and closed with 60 to win the Evian Championship.
"I was so amazed, and I asked my caddie why I'm playing well this week," Ryu said. "It's because I'm more calm and enjoying the golf more. That's why these days my golf is going pretty well."
Korda has work to do. The dominant player through the first week of June, the American star missed the cut at Evian Championship and she couldn't atone for four bogeys in a pedestrian round of 73.
That was still better than most of the Australians, with Cassie Porter's one-under 70 the best of a mostly bad bunch.
Hannah Green and Robyn Choi both carded level-par 71s but the rest have second-round work to do to make the halfway cut.
Minjee Lee and Steph Kyriacou were both two under on the front nine before crashing and signing for 75s, a four-over score Grace Kim matched.
Karis Davidson (76), Kelsey Bennett (77) and Justice Bosio (79) are also well off the pace.
Thitikul had her best chance to win a major at the Evian a year ago when Aussie Kim pulled off a stunning playoff victory. She's yet to contend in a major this year, but is off to a great start in the final one.
"I knew that I haven't won one yet, but I feel like I'm so young and then I still had I don't know how many majors ahead of me," Thitikul said. "I feel like one day it will come if I do my job."
Kuwaki, who has won twice this year on the Japan LPGA, playing in only her seventh LPGA major. She also had three straight birdies on the front nine and dropped only one shot.
Celine Boutier and Yealimi Noh each shot 67 to be three behind. Charley Hull, another player often reminded she has yet to win a major, was in the large group of players at 69.
With AAP
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