MOBILE, Alabama, May 16, 2013 (AFP) - Thompson, trying to improve on her runner-up finish in the same tournament last year, had eight birdies and a bogey.
They were one stroke in front of Jessica Korda, who had six birdies in her six-under 66.
Nicole Castrale, Mina Harigae, Dewi Claire Schreefel, Thidapa Suwannapura Park Hee-Young, Jennifer Johnson and Chella Choi were tied on 67.
Defending champion and world number two Stacy Lewis, looking for a solid showing to gain ground on absent world number one Park In-Bee, carded a two-under par 70 that gave her a share of 33rd place.
Ji, the 2009 US Open champion, made five birdies on the front nine.
"I felt really good with my putting today," she said. "If I had five- or 10-foot putts, I felt like I could make it. So I was really focused on it and I was really positive with my putter."
Ji got to seven-under with her last birdie of the day, on the par-five 16th. She left a 15-foot birdie attempt at 18 just short.
"I just like a really fast green," she said. "This is really fast and in really good shape on the course. I just like it."
Thompson teed off on 10 and nabbed her first birdie of the day at the par-five 13th.
"I love the par-fives here, I get to hit pretty much driver on every hole out here so it's definitely an advantage for me," said Thompson, who birdied three of the four par-fives.
She made the turn at four-under, and after her lone bogey of the day at the par-three second hole she strung together three birdies from the fifth hole, and added one more at the ninth.
Lewis capped her round with a double-bogey at the par-four ninth hole, where she hit her tee shot into the trees.
"I didn't really have anything go right today," said Lewis, who re-teed and the fired to the base of a grandstand. "I had one ball hit the sprinkler and go in the trees and one hit the cart path and go in the trees. Just kind of got some bad breaks with lies and really just didn't have any bounces go my way. So I guess to shoot what I did was pretty good."
Leading first-round scores on Thursday in the Mobile Bay LPGA Classic (USA unless noted, par-72):
65 - Ji Eun-Hee (KOR), Lexi Thompson
66 - Jessica Korda
67 - Nicole Castrale, Mina Harigae, Dewi Claire Schreefel (NED), Thidapa Suwannapura (THA), Park Hee-Young (KOR), Jennifer Johnson, Chella Choi (KOR)
68 - Sandra Gal (GER), Beatriz Recari (ESP), Marcy Hart, Lauren Doughtie, Amelia Lewis, Vicky Hurst, Seo Hee-Kyung (KOR), Dori Carter, Nicole Jeray
69 - Nicole Hage, Julieta Granada (PAR), Pornanong Phatlum (THA), Ariya Jutanugarn (THA), Paz Echeverria (CHI), Lisa McCloskey, Nicole Smith, Kim Song-Hee (KOR), Alison Walshe, Stacy Prammanasudh, Katherine Hull-Kirk (AUS), Karrie Webb, Kristy McPherson