The former Georgia Tech star ran off three consecutive birdies late in his round for a 5-under-par 66.
Billy Horschel, who tied for fourth in the real U.S. Open two weeks ago at Merion, began his day with a 50-foot birdie putt, added a pair of birdies over the next three holes and then hung on for 68, which Bud Cauley matched.
The 7,569-yard course is among the longest this year. The rough is denser than usual, putting an extra premium on the tee shot.
Jim Furyk, Nicolas Colsaerts and Ben Kohles were at 69.
Davis Love III had 83 with a sore hip and then withdrew, not wanting to risk further injury.
Rory Sabbatini withdrew with a sore back after he was 8-over in 12 holes. Charlie Beljan had 84.
Lucas Glover, a former U.S. Open champion, called it “the most boring round of PGA Tour golf I’ve heard.”
“I heard two cheers across the whole golf course all morning,” Glover said after a hard-earned 71. “They definitely weren’t for my group.”
Castro played his first U.S. Open last year at Olympic Club and missed the cut. He saw similarities in one respect.
“There’s not a lot of birdies,” he said.
“Two U.S. Opens in three weeks,” said George McNeill, who had a 71.
CHAMPIONS TOUR: In Pittsburgh, John Huston shot 5-under-par 65 to take the first-round lead in the Senior Players Championship.
Fred Couples, Duffy Waldorf, Fred Funk and Russ Cochran were a stroke back in the third major on the Champions Tour schedule. Tom Pernice Jr. had three consecutive birdies to start his round and led a group of players at 67.
Augusta native Larry Mize opened with 68.
Scotland’s Colin Montgomerie fired 69 in senior debut.
Huston, a seven-time PGA Tour winner got his round going with a tap-in birdie on the par-5 18th, kickstarting a stretch where he made five birdies in a span of seven holes, including a 40-footer on No. 6.
WEB.COM TOUR: In Newburgh, Ind., Patrick Sheehan and John Peterson shot 6-under-par 66s to share the first round lead in the United Leasing Championship.
Kevin Tway and Adam Crawford are tied for third place at 67.
Aiken’s Kevin Kisner shot 72.
EUROPEAN TOUR: In Maynooth, Ireland, Oscar Floren shot 6-under-par 66 to lead the Irish Open.
He’s one ahead of Shane Lowry, Jean-Baptiste Gonnet, Joost Luiten, Michael Hoey and former Oklahoma State golfer Peter Uihlein.
Rory McIlroy shot 74.