Rosales, the 24-year-old five-time RP Ladies Open champion, birdied the par-3 15th and the par-5 17th, cushioning three bogeys in the frontnine and two more in the backnine to be among the top 50 scorers at the start of the fabled tournament featuring 150 players.
Only six shots behind leaders Laura Diaz, Shani Waugh and Juli Inkster, Rosales could easily make the cut if she sustains her game in the second round Friday.
Having a lot more catching up to do was Fil-Am bet Dorothy Delasin who struggled with a six-over 76 for a piece of 90th place. She was nine shots off the pace.
Delasin was tied with Michele Fuller, Eva Dahllof, Namika Omata, Karine Icher, Natalie Gulbis, Becky Iverson, Amy Fruhwirth, Sue Daniels, Kristal Parker-Manzo, Betsy King and Patricia Meunier Lebouc.
Rosales proved to be steadier at the start of the $3 million tourney with a 12-of-14 fairway hit percentage and a 1.67-putt average. Her average drive was 272.5 yards.
The one-time US NCAA champion was tied with Carin Koch, Wendy Doolan, Gloria Park, Lynnette Brooky, Courtney Swaim, Leslie Spalding, Allison Finney, Laurel Kean, Meg Mallon, Jeong Jang, Charlotta Sorenstam, Sherri Turner and Ai Siew Lim at three-over.