Castrale, a Southern California native, fired a 4-under 68 on Saturday to take a two-shot lead over Hee-Won Han heading into the final round of the Honda LPGA Thailand.
She is 11-under-par 133 through two rounds at Amata Spring Country Club, which is hosting the first-ever LPGA event in Thailand.
Meanwhile, Castrale is leading for the first time heading into the final round.
'I can play well and possibly not win,' she said about Sunday. 'Hopefully that doesn't happen, but hopefully I play well and see what happens. That is all I can really do.'
Castrale, 27, turned professional in 2001 but hasn't played much on the LPGA Tour. She won back-to-back tournaments on the Duramed Futures Tour in 2005 and finished fourth on the money list to earn her exempt status for this year's LPGA season.
She had been competing on the Futures Tour since losing her LPGA status following the 2003 season.
'I pretty much have stuck to the same game plan [this year] and I felt like sooner or later things would come together,' said Castrale. 'Hopefully this is the week that it all comes together.'
Saturday, Castrale successfully negotiated lakes, rivers and considerable elevation change at the challenging par-72 layout to record six birdies and two bogeys.
She began her second round with four consecutive birdies, making putts between 3 and 20 feet, and was 12 under around the turn after making another birdie at the seventh.
Castrale was as low as 13 under after draining a 15-foot birdie putt at the 12th, but she stumbled to a pair of tough bogeys at 13 and 16.
Still, Castrale managed to make her second consecutive par at the unique 17th hole, featuring Asia's only true island green, and at the 18th.
'The back nine definitely got windy, but I still think I played pretty good,' said Castrale. 'I am definitely happy with the outcome.
'(Tomorrow) I am pretty much going to stick to the same game plan. I feel like I'm playing pretty well right now. I am just going to go back to trying to execute shots and hit fairways and greens and hopefully make the putts from there.'
Han also shot a 4-under 68 and stands alone in second place at 9-under 135. Christina Kim had a 6-under 66 and is third at 8-under 136.
Gloria Park (68) and Diana D'Alessio (69) are one shot further back at 137.
Heather Young, a co-first round leader with Castrale, shot a 1-over 73 and fell into solo sixth place at 6 under.