Castro's best finish on the PGA Tour has been a tie for seventh at last year's Greenbrier Classic. He got his round going with three straight birdies on the front nine and put himself in position to challenge for his first victory.
Castro is barely inside the Top 100 in the FedEx Cup points standings this year and early in the season missed the cut in six out of eight appearances.
He then made six cuts in a row, although his best finish in that stretch was a tie for 18th at the St. Jude Classic in Memphis.
Lytro unveils camera that can focus a photo after shooting it
Easer Egg Roll brings thousands to White House