PONTE VEDRA BEACH, Florida, May 9, 2013 (AFP) - Castro, making his debut in the US PGA Tour's $9.5 million flagship event, equalled the record set by Fred Couples in 1992 and matched by Australian Greg Norman in 1994.
His effort gave him a three-shot lead over world number two Rory McIlroy and American Zach Johnson, who shared second place on six-under 66.
Northern Ireland's McIlroy, who failed to make the cut in three prior appearances in the tournament at TPC Sawgrass, said he benefitted from perfect scoring conditions.
"It was benign out there," he said. "There wasn't much wind. It was perfect conditions for scoring and I was able to take advantage of those."
World number one Tiger Woods, who won the title in 2001, bogeyed his final hole to fall into a group of players sharing fourth place on five-under 67.
He was joined by Steve Stricker, Hunter Mahan, Webb Simpson, Casey Wittenberg and Ryan Palmer.
Castro's round included seven birdies and an eagle. He teed off on 10 and gabbed his first birdie at the 11th, where he drained a seven-foot birdie putt.
After another birdie at 14, he reeled off three straight birdies from the 16th. He eagled the par-five second hole, where he stuck his second shot within four feet.
Two more birdies, at the fourth and sixth, saw him match the record.
"It was a fun day," Castro said. "(It's) nice to have 18 holes that I showed what I've been working on."
McIlroy also played without a bogey, picking up six birdies before finishing out his round with seven straight pars.
"I'm definitely a lot more relaxed coming in here this year," McIlroy said. "Whatever I do this week, what I felt coming in is I'll do better than I ever have before.
"I feel like I've got the game to contend. I just wanted to go out and play well, and that's what I've done so far."
Johnson had seven birdies and one bogey to join McIlroy.
Woods's six birdies included four in a row starting at the ninth, with his only miscue a bogey at 18, where his second shot rolled off the back of the green.
"Overall, I think I need to strike the ball a little better than I did today," Woods said.