Prammanasudh rolled in a 12-foot eagle putt on the par-5 eighth hole at Capitol Hill's Senator Course en route to a 4-under-par 68 that left her at 13-under 131, three strokes ahead of Ochoa.
The Mexican, seeking her fourth straight victory, finished with a 68. After her round, LPGA rules official Jane Reynolds was summoned to the scoring tent about an incident on the second hole in which Ochoa's ball oscillated on the green as she went to tap it in for a par.
'I didn't cause the ball to move because I didn't ground my club, so there was no penalty,' Ochoa said. 'I kind of knew but I just wanted to get word from an official to make sure everything was straight before I signed my scorecard.'
After opening with eight straight pars, Ochoa was six strokes behind Prammanasudh, who played in the morning.
'I knew right away that it would be important to keep making birdies and get as close as I can,' Ochoa said. 'I think I'm in good shape. We still have two more days. I like my position and it's about putting pressure on her and having a good round tomorrow.'
Ochoa birdied Nos. 9 and 10 before her first bogey of the tournament at No. 14.
'I was upset and told myself ... to finish strong and get in that last group for tomorrow,' Ochoa said. 'My caddie told me that we were fine and the pin positions on the last three holes were easy and on the front and good birdie chances to finish strong and get in a good position.'
Prammanasudh is coming off a hectic schedule in which she played the Solheim Cup in Sweden, attended a corporate outing in Monte Carlo and was one of several Thai athletes honored in New York on Sunday by Thai Prime Minister Surayud Chulanont.
'Obviously, I made a lot of putts yesterday,' said Prammanasudh, who matched a career low with a 63 Thursday. 'I was still hitting some good putts today. They just didn't fall like they did yesterday. Luckily the eagle fell, so I couldn't be happier.
'It feels great, you know, being this late in the year and playing this well. I couldn't be more pleased. There still are two more rounds and the birdies are still out there definitely if somebody gets hot. I've just got to keep doing what I'm doing and hopefully stay on top.'