England's Karen Stupples produced a late birdie spree to move into the top five at the Safeway International in Phoenix, Arizona - but it was American Angela Stanford who set the tournament at Superstition Mountain alight.
Stupples carded a five-under-par 67, while Stanford signed for a course-record 62.
Stanford is three shots clear of Mexico's defending champion and world number one Lorena Ochoa, who hit nine birdies and two bogeys in her 65.
Stupples, the 2004 Women's British Open champion, missed most of last year following the birth of her son, Logan, in April.
But she showed some fine form towards the end of last season and finished tied fifth in the Fields Open in Hawaii last month.
"Getting back my fitness was the most difficult part in coming back after having Logan," said Stupples.
"But I'm pleased the way I've started this year and delighted with this round, especially the finish."