After successive tap-in birdies at 15 and 16, Thompson needed to make a much longer effort at the 18th to pick up the two shots she required to snatch the lead outright at the Mission Hills Country Club in Rancho Mirage, California.
The 21-year-old American, winner of the title in 2014, successfully drained the putt to finish with a three-under-par 69 and 54-hole total of 10-under 206 at the first major of the LPGA season.
“I was playing a little bit timid,” Thompson told Golf Channel.
“To finish like that definitely turned around my confidence.”
There are 10 golfers within three shots of Thompson on a crowded leaderboard. Thai Ariya fired the low round of the day with a 67, while Ko and South Korea’s Chun each carded 69s to move to nine-under.
England’s Charley Hull (69) and Japan’s former world number one Ai Miyazato (71) were tied for fifth at eight-under.
German Caroline Masson (68), American Michelle Wie (70), South Korean Park Sung-hyun (71), Norway’s Suzann Pettersen (71) and South African Lee-Anne Pace (71) were all at seven-under.
The 18-year-old Ko is vying to become the youngest player to win two women’s majors, a record set by Pak Se-ri when she won the LPGA Championship and U.S. Open at the age of 20 in 1998.
New Zealander Ko won the final major of 2015, the Evian Championship, and came into the ANA Inspiration on the back of her 11th LPGA title at last weekend’s Kia Classic.
“I felt that I struck the ball much better today and I hit a few more fairways,” Ko said of her bogey-free round.
“The less time you spend in the rough the better. I feel I was able to control my shots and I have an idea where the ball was going to go, which especially around a course like this it’s good to have that.”
Original source: Hot finish gives Lexi Inspiration-al lead