Castro bounced back from that early bogey on No. 11 with birdies on the next two holes, and then he ran off three straight birdies toward the end of his round that required some highly skilled shots — a 20-foot putt on No. 5, a perfect 3-wood to about 15 feet for a two-putt birdie on the par-5 sixth, and a chip-in just off the seventh green.
But it was a quiet day for the most part.
"Two U.S. Opens in three weeks," said George McNeill, who had a 71 while playing with Jonas Blixt and Ben Curtis. "And before that, we got to play the U.S. Open at Muirfield (Village), too. It was fairly quiet out there. You have a few cheers here and there. But we had the 'hot dog' group. That's where the fans are looking at the pairing sheet and go, 'Curtis, Blixt, McNeill. Let's go get a hot dog.'"
It didn't help that Tiger Woods wasn't around, unable to play because of a sore left elbow that will keep him out of competition until the British Open next month.
Woods won last year at 8-under 276, one of the higher winning scores on tour in 2012.
"You don't usually see first-round scores on a PGA Tour event only be 3-under leading after the morning wave," Horschel said. "It shows you how tough this golf course is, shows you how long the rough is."
It's the second time in the last seven weeks that Castro got off to a great start. He had a course record-tying 63 on the TPC Sawgrass for a three-shot lead at The Players Championship, and he wasn't sure which was tougher.
"They were totally different rounds," he said. "The one at Sawgrass, I hit it 3 feet eight or nine times. And the one today was more of a normal, lower round where I made some putts. It's hard to compare these two golf courses. That one was playing firm and fast. This one is just long and soft."