Rosales hit a seven-under par 65 for a three-round score of 12-under for a share of the lead alongside Sweden’s Carin Koch who carded a third successive four-under par 68.
Rosales stormed up the field with an error-free front nine of 32, with birdies at the third, sixth, seventh and ninth, adding another at the tough par-three 11th from eight feet.
At the 16th, the 23-year-old from Manila rolled in a nine footer and equalled Candie Kung’s course record with a six-foot putt at the 17th.
"I’m really surprised," said Rosales, who could become the first player from the Philippines to win a major championship.
"I felt really good this week hitting my drives in the fairways and being where I want to be and hit as many greens as I have done."
Having never been in the final round before, the diminutive America-based player admitted to being more excited than nervous.
"I am looking forward to getting experience and tomorrow, we’ll see what happens."
Sharing third place on ten under par are 19-year-old Natalie Gulbis from the USA, who moved up with a five-under-par 67 in the sunny conditions and 22-year-old Paula Marti from Spain who hit five birdies during her 69.
Five players share fifth place on nine under par including the two-time winner of this event, Karrie Webb from Australia, who, if she wins, will become the second youngest player in history to win six major championships. (AFP)