World number two Jin Young Ko leads the AIG Women's Open by two shots after a first-round seven-under-par 64 at Royal Lytham & St Anne's.
Thitikul, who has 11 top-10 finishes in majors without a victory, played a bogey-free round. South Korea's Haeran Ryu and Japan's Shiho Kuwaki are tied for second place at five under par.
Ryu is seeking to become only the fifth woman in history to win three majors in a calendar year.
Notable players further down the leaderboard include American Yani Noh and France's Celine Boutier at four under. Japan's Sakura Koiwai and Thailand's Atthaya Thitikul are at three under. England's Lottie Woad and Charley Hull are both on two under, while compatriot Alice Hewson finished her round at one under.
World number one Nelly Korda is among the players who fell behind, finishing her opening round at two over par with four bogeys.
"Winning one [major] doesn't mean I am going to have a really good, successful career," Thitikul said. "Winning one doesn't mean much, I would rather be a consistent player, more than just winning one and then gone. Sometimes I had a hard time dealing with all the noise."