Henderson, from Smiths Falls, Ont., shot a tournament-record 7-under 65 to take the lead halfway through the second round.
"That's pretty cool," the 17-year-old Henderson said.
That sizzling round put her at 9 under at Lake Merced Golf Club, where world No. 1 and defending champion Lydia Ko shot an even-par 72 to remain 5 under after taking the first-round lead Thursday. There were still afternoon rounds to play Friday, including local favourite Juli Inkster after her opening 68.
As Ko celebrated her 18th birthday, she cleared the way for a new teen star to shine.