Facing the infamous island-green on No. 17 for the first time in competition, Castro left a 9-iron within a foot of the hole. On the daunting 18th, he hit 4-iron to a foot. When his day was over, Castro was in the record book with Greg Norman and Fred Couples for lowest score on the Stadium Course.
"I hit it close a lot," said Castro, making it sound as easy as it looked.
Sawgrass finally made sense to McIlroy, too.
The two-time major champion had never made the cut or even broken par at The Players Championship, but the 24-year-old figured it out on a gorgeous morning by dialing it back off the tee and letting his iron play take over. McIlroy never came seriously close to a bogey on his way to a 66, leaving him tied with Zach Johnson.
McIlroy didn't hit driver once on the front nine, including the par-5 ninth. He made up his mind during practice that he gets into more trouble trying to reach the green in two, so he is playing short and relying on his wedge. He still made par on No. 9, but he stuck to his strategy.
"When you hit the shots, it seems very simple," McIlroy said. "I had a lot of good shots out there, lot of iron shots that were 12, 15 feet away from the pin and I got myself a lot of looks for birdies. I adopted maybe more of a conservative strategy off the tee this year. But once you put your ball in the fairway that means you can be more aggressive into the greens. So it sort of balances itself out."
Among those playing in the afternoon were Tiger Woods, Phil Mickelson and Vijay Singh, who is under more scrutiny than ever. Singh shook the tournament on Wednesday when he filed a lawsuit against the PGA Tour for what he said was a rush to judgment in trying to suspend him for an anti-doping violation. Singh sued just one week after the tour dropped its case.
One man standing in the bleachers around the first tee was wearing deer antlers. Singh confirmed using deer antler spray, which led to the tour's investigation.
Masters champion Adam Scott didn't miss a beat from his last competitive round, a rainy Sunday at Augusta National. He missed only two fairways and two greens in a tidy round of 69. His only disappointment was not getting introduced as the Masters champion.
Scott and McIlroy were joined by Steve Stricker, who made six birdies and never once had honours on the tee — Stricker and McIlroy made birdies on the same six holes. Stricker wound up with a 67, along with Hunter Mahan and Casey Wittenberg.
Henrik Stenson was among those at 68.
The Stadium Course has rarely been this vulnerable, with barely a trace of wind and some pins in bowls that allowed for good looks at birdie. Half the 72 players in the morning broke par.
But the punishment is never far away, as Scott Stallings discovered. He opened with five straight birdies to get everyone's attention, but after going out in 31, Stallings gave most of it back with a bogey, double bogey and a triple bogey on the 16th when he hit two balls into the water. He shot 40 on the back for a 71.
"It just goes to show about the golf course and really how volatile it is," Stallings said.
Castro hasn't discovered that yet. This was a day when everything went right.
He made three birdies early in his round on the back until making his tap-in birdies on the 17th and 18th. Then, he hit 3-iron into 3 feet on the par-5 second hole for an eagle and was 7-under through 11 holes. On the fourth, he hit his approach inside 2 feet for another birdie, and then he hit wedge to 18 inches on the sixth.
Castro had a birdie putt just outside 12 feet to break the course record, but missed it.
After his eagle on the second hole — his 11th of the round — Castro was reminded of a game he plays to try to birdie every other hole.
"When I eagled, I was like, 'Oh, now I'm 7 (under) through 11, so that's keeping ahead of that 50 per cent mark.' But this is a different golf course," he said. "The golf course is much harder than most of the other times I've kind of taken that attitude."
Couples shot his 63 in 1992 in the third round. Norman opened with a 63 in 1994 when he went on to shatter the tournament record at 24-under 264. Castro put himself in good company, two players in the World Golf Hall of Fame, and two winners of The Players Championship. Not bad for a 27-year-old in only his second year on the PGA Tour.
Castro is not well known even in golf circles. His mother grew up in Peru and moved to America as a teenager along with her sister, Jenny Lidback, who played the LPGA Tour. He toiled in the minor leagues for five years after getting his industrial engineering degree at Georgia Tech.
And he's in the record book at The Players Championship. Sawgrass is rarely in short supply of surprises.
"It gets proven out here every week," Castro said. "Derek Ernst won last week in his eighth event and no one ever heard of him. There are a lot of good players out here."