Ochoa posted a 4-under 68 on Saturday and is one ahead of Catriona Matthew, who carded a 7-under 65 in the third round. Matthew is three back at minus-8.
Christina Kim shot a 3-under 69 in round three and is alone in fourth place at 6-under-par 210. Cristie Kerr, who was part of a logjam atop the leaderboard after the second round, posted an even-par 72 and is tied for fifth with Grace Park and Jeong Jang, who both put together rounds of 2-under 70. The trio is knotted at 5-under-par 211.
Prammanasudh, who trailed the four second-round co-leaders by a single shot, took some time to break into red figures in her third round. She parred her first five holes, then knocked a 4-iron to 5 feet to set up birdie at the sixth.
Prammanasudh got on the par-5 seventh green in two with a 5-wood and two-putted from 35 feet for back-to-back birdies. The 25-year-old tallied her third birdie in a row at the eighth when she rolled in a 25-footer.
At the 10th, Prammanasudh hit an 8-iron to 8 feet and she converted the birdie putt. She drained a 6-footer for birdie at the 15th which tied her for the lead with Ochoa, who was already in the clubhouse.
Prammanasudh had only a pitching-wedge for her second at the short, par-4 17th. Her approach stopped 6 feet from the cup and she sank the birdie try for sole possession of the lead.
She birdied the closing hole at the Ironhorse Course at Vanderbilt Legends Club to take a two-shot lead. Not only will Prammanasudh be in search of her first LPGA Tour victory, but Saturday marked the first time in her young career that she held the lead with one round to play.
'It's a feeling I've never had to experience before,' said Prammanasudh. 'But, I've got another 18 holes and five hours. There a lot of tough competitors out there, so it will be tough.'
Directly in Prammanasudh's rear-view mirror will be Ochoa. When Prammanasudh was a senior at the University of Tulsa, Ochoa was wrapping up her collegiate career at Arizona State University. Ochoa left after her sophomore season and was ranked No. 1. Prammanasudh was No. 2, but now she can exact some revenge.
'She's going to play her game and I'm going to stick to my game,' said Prammanasudh, referring to last year's winner. 'I just know that she is mentally tough and that I have to go out and beat her and whoever else.'
Ochoa, one of the four second-round co-leaders, collected four birdies in her first 11 holes, all from outside 10 feet. The defending champion hit a 4- iron over the green at the 13th, but chipped inside 4 feet. She missed her par putt to fall to 8 under par for the championship.
Ochoa reclaimed the dropped shot at the very next hole. She hit a 7-iron to two feet with her third at the par-5 hole and tapped in for her fifth and final birdie of the round.
'I feel confident about my game and I'm going to come out tomorrow and play my game,' said Ochoa. 'I'm going to do the same thing I've been doing. It is a special place for me, kind of like home. I hope I can keep that feeling going.'
Giulia Sergas (70) and Heather Bowie (72) share eighth place at minus-four, while Karrie Webb (72) is alone in 10th place at 3-under-par 213.
Liselotte Neumann and Wendy Ward were part of the group that shared the second-round lead, but both struggled to rounds of 4-over 76 and are tied for 13th place at minus-1.