Dina Ammaccapane, Nina Reis and Brittany Lang are knotted in second place at 6-under-par 66.
Lorena Ochoa, the LPGA Tour's money leader, carded a 5-under-par 67 on Thursday and is part of a large group tied for fifth place.
Ochoa, who earned her third victory of the season last week, collected three birdies in her opening five holes, but then parred her final four holes on the front side.
The tour's leading money winner birdied the 10th, but dropped a shot on No. 13. Ochoa moved up the leaderboard with a pair of birdies at 16 and 18 and now is in a strong place to keep her lead on the money list.
'I played very good,' said Ochoa. 'I think I'm in a good position for the next few days and that is all that matters. My game is good and I'll be ready to go tomorrow.'
Annika Sorenstam, who played with Ochoa on Thursday, shot a 2-under 70 and is part of a group tied for 26th place.
Sorenstam birdied the first from 3 feet, and eagled the par-5 fourth. She bogeyed five, but got that shot back with a birdie at No. 8.
The Swede birdied the 14th hole, but she bogeyed 16 when her approach landed in the water. Sorenstam also bogeyed the last to fall down the leaderboard.
'I'm not really worried,' acknowledged Sorenstam, who has never won this title. 'I'm just disappointed obviously. It's no fun to finish the way I did.'
Sorenstam is looking up at her Solheim Cup teammate.
Hjorth began on the back nine Thursday and collected her first birdie at the par-5 12th when her 8-footer found the bottom of the cup. She tallied one other first-nine birdie, at the par-5 15th. Hjorth came up short of the green with her second, then chipped to 4 feet to make the turn at 2 under par.
The Swede parred her first three holes on the second nine, but caught fire.
At the par-5 fourth, Hjorth hit her rescue-club over the green, but chipped inside a foot. She made it back-to-back birdies when she hit a 7-iron to 20 feet at the par-3 fifth.
Hjorth kept her strong play going at six. She drove into a fairway bunker, but knocked an 8-iron to 15 feet to set up her third birdie in a row. She polished off her fourth birdie in a row at seven when she played a 6-iron inside 5 feet.
'I was hitting a few close and then also making the putts that I needed,' said Hjorth, referring to her birdie run on the second nine. 'It was fun.'
Hjorth parred the eighth, but closed in style. At the par-4 ninth, Hjorth knocked a 7-iron to 20 feet and converted the birdie putt to grab sole possession of the lead.
'I played pretty well,' said Hjorth, whose best finish this season was at her first event, a tie for 13th at the SBS Open at Turtle Bay. 'I played very solid all day. Hopefully I can keep going. I've had some good rounds, but not really four in a row. Hopefully this week will be the week to put it all together.'
Sophie Gustafson, Julieta Granada, Il Mi Chung, Young Kim, Seon-Hwa Lee, Kellie Kuehne and Catherine Cartwright joined Ochoa in fifth place at minus-5.
Defending champion Pat Hurst, who lost to Sorenstam in a playoff at the U.S. Women's Open, struggled to a 1-over-par 73 and is tied for 85th place.