Tseng struck a six-iron to four feet to set up an eagle at her 17th hole, the par-five eighth, and then had to wait out a 96-minute weather delay due to the threat of lightning before returning to the course to pocket her sixth birdie of the round to post an 10-under total of 134.
That left her a stroke in front of American Austin Ernst, who had carded a 65 earlier in the day in pursuit of her second victory on the LPGA Tour.
Ernst's playing partner, American world number eight Lexi Thompson, was a further stroke back at eight under after carding a bogey-free 67 that included four birdies in her first eight holes.
Several players will complete the round on Saturday morning, though Tseng's lead appears safe, with nobody among those yet to finish better than seven-under.
Tseng has not won an LPGA event since 2012 but feels that she is trending in the right direction to claim her 16th career victory soon.
"I've been working on my game forever, like every day," she told reporters.
"It's just exciting. (I am) just very happy my game's really coming back... I want to win... It will come. It doesn't matter if it's this week or next week or next year."
Behind Tseng, Ernst and Thompson spurred each other on to make a combined 13 birdies and an eagle.
"We both had a string of birdies there on the front nine," Ernst told reporters.
"She was hitting it well. Definitely when you see putts going in with other people, too, I think it just kind of gives you a little bit more confidence that you can just take it deep."
Thompson is relishing being back on the Senator Course where she claimed her first LPGA win in 2011 before finishing runner-up at the venue the following year.
"It helps knowing the golf course really well," said the 20-year-old, who is already a five-times winner on the circuit. "I always love coming to Alabama and playing this golf course
(Reporting by Mark Lamport-Stokes in Los Angeles and Andrew Both in Cary, North Carolina; Editing by John O'Brien)
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