She will have familiar company in the final group Sunday.
Joining Sorenstam in the lead was Paula Creamer, who challenged the Swede over a drop at last year's ADT Championship, famously calling into question the conscience of the world's No. 1 women's player.
Creamer fired a 5-under 67 in the second round, while Sorenstam birdied a hole late and finished with a 1-under 71 on the day. Korean Mi Hyun Kim is also at minus-6 after a 69.
Sorenstam, who shared the overnight lead with Italian Silvia Cavalleri at 5 under, moved further into red numbers with birdies at the third and fourth holes. Both putts were from inside 10 feet.
Five straight pars were followed by consecutive bogeys at 10 and 11, which dropped Sorenstam back to 5 under and out of first place.
At the par-4 10th, she three-putted. At the par-3 11th, her tee shot missed the green.
Sorenstam got one stroke back at the par-4 16th, where she grooved a 7 iron within 6 feet to set up a birdie try. Despite the waning daylight at Mexico City's Bosque Real Country Club, Sorenstam was able to roll in the putt.
'I thought I played quite well today,' said Sorenstam, who won this event last year for her first of 10 titles in 2005. 'I got off to a really good start. There were some things in the middle -- I couldn't put a good score together -- but I played really, really well.
'I'm a little disappointed [that] in the end it was getting so dark. I couldn't really read the greens for the last few holes, so that was a bummer.'
Creamer's round was a little less up-and-down. She began steadily with five straight pars until a birdie at the par-5 sixth moved her to minus-2.
A bogey at 10 was followed by consecutive birdies at 12 and 13, and an eagle at the par-4 15th, where she one-bounced a pitching wedge shot into the hole from 120 yards out.
'After [the bogey at 10] I kind of got excited and got a little energy into me,' said Creamer. 'Even though it was negative energy, I still had some more momentum. I played solid on the back nine.'
Standing one shot off the lead is Seon-Hwa Lee, who fired a 3-under 69 to reach minus-5. Candie Kung (68), Siew-Ai Lim (68) and Cristie Kerr (69) share fifth place, one stroke further back.
Cavalleri, the co-first round leader, carded a 5-over 77 and tumbled into a tie for 18th place at even-par.
The cut fell at plus-5, with 72 players moving on to the final round of this 54-hole event. Among those advancing were three Mexican amateurs playing on sponsor's exemptions.
The second round was delayed at the beginning by frost on low-lying areas of the course, and it was completed as darkness fell.