At one point Lee held a four-shot advantage over the chasing pack, having headed into Saturday's running two clear. However, a bogey on the par-4 16th and a double bogey five at the par-3 17th saw the American stutter to a round of 71 and a score of 10-under.
Delacour, who is searching for her first LPGA Tour title, made the most of Lee's troubles. After taking the turn in 34, Delacour dropped only the her third shot of the week at the 11th, but the Frenchwomen bounced back strongly, birding three of the last five holes in a round of 67.
Two shots off the pace are Paula Creamer, So Yeon Ryu, and Minjee Lee who carded 66, 68 and 66 respectively. Creamer was one of the big moves on the penultimate day, with an eagle at the third jump-starting her round.
A further shot back is Lexi Thompson on eight-under, while world number one Lydia Ko is ten strokes behind Delacour after a third-round 70.