Ochoa, who lost a playoff to Karrie Webb two weeks ago at the Kraft Nabisco Championship, equaled her own course record at Las Vegas Country Club on Thursday. She originally set the mark in the second round last year.
This is a rare 54-hole event that begins on Thursday and ends on Saturday.
Ochoa, who registered a double-eagle in Wednesday's Pro-Am, started poorly on Thursday. She knocked her second shot over the green at the par-4 first, and two-putted for bogey from 5 feet. That would be the last blemish on the scorecard for the fifth-ranked player in the world.
At the par-3 third, Ochoa hit a 7-iron to 10 feet to set up birdie. She made it two in a row thanks to a beautiful wedge shot at the fourth that left Ochoa with a tap-in birdie.
Ochoa recorded her first eagle at the par-5 sixth when she hit a 5-wood to the back fringe with her second shot. She rolled in the 30-footer and parred the next three holes to make the turn at 3-under-par 33.
Ochoa picked up where she left off on the back nine. She two-putted for birdie from 35 feet at the par-5 10th, then wedged her approach to 12 feet to set up another birdie at No. 11.
She parred the 12th, but hit a pitching-wedge to 3 feet at 13. Ochoa rolled in the short birdie putt, then collected her second birdie in a row at the par-3 14th. Ochoa pulled an 8-iron and landed the ball 13 feet from the hole. That birdie putt moved her into a tie for the lead at minus-7.
Ochoa's second eagle of the round came at Las Vegas Country Club's closing hole. At the 495-yard hole, Ochoa hit a 5-wood to 12 feet and drained the eagle putt to move into sole possession of the top spot on the leaderboard at 9 under par.
'It was a really good day,' acknowledged Ochoa, a three-time winner on the LPGA Tour. 'I hit all of the fairways, and I keep looking at the flag and making putts, so it was a great run. I enjoyed it.'
Ochoa is not just riding a hot streak from her play two weeks ago. She fired a final-round, 7-under 65 last year, but ultimately came up two strokes short as Wendy Ward went on to the winner's circle.
'I enjoy playing here,' said Ochoa. 'The conditions were perfect in the morning. It is a tricky golf course. You have to think about the hole and make the right decision on the tee and get the right line.'
Ahn was inconsistent on the front nine as she traded three birdies and two bogeys. On the back nine, Ahn used an amazing run, starting at the 11th, to soar up the leaderboard.
Ahn rolled in four straight, 12-foot birdie putts. At the par-4 15th, Ahn hit a pitching-wedge to 10 feet to set up her fifth consecutive birdie. Her final birdie came at 16 when her 7-iron stopped nine feet from the flagstick.
'I wasn't trying to make birdies,' admitted Ahn, whose only win on the LPGA Tour came at the 2003 CJ Nine Bridges Classic. 'I was just trying to stay calm and focus, then play it shot-by-shot.'
Ward opened her title defense impressively with a 5-under-par 67. She is tied for third place with Teresa Lu, Miriam Nagl, Jee Young Lee, Maria Hjorth, Jimin Kang, Mi Hyun Kim, Ashli Bunch and Seon Hwa Lee.