Tiger Woods and Rory McIlroy strode shoulder to shoulder at just about a quarter to 4 on Friday afternoon, their work for the day done, the time for chitchat — not to mention food and rest — finally upon them. They walked from the 10th green at Merion Golf Club’s East Course, where they had just completed the last of their 25 holes for the day, and took a path across the first fairway en route to the scoring tent.

As they walked and fans squealed their names, Phil Mickelson left the first tee, heading toward his ball in the right rough. Woods and McIlroy, the two top-ranked players in the world, didn’t cross Mickelson’s path with sexy numbers on their scorecards, each at 3-over-par 143 over two rounds.

But they could well have issued a warning, right then and there. At that hour, Mickelson led the U.S. Open by virtue of his opening 67. But Woods and McIlroy and anyone who had finished understood the arduous task ahead. Midway through the 113th playing of this event, Merion is more than proving that, for a little place, it is a lot of golf course.

When Mickelson finally finished nearly five hours later, his lead was gone, but his swagger remained. The U.S. Open — with all the frustration and fascination it brings — was on.

“I just like being in the mix,” he said.

It is a mix that includes so much intrigue. With two days to go, Merion has allowed precisely two of 156 players to break par. They are Billy Horschel, a 26-year-old playing his second major who somehow hit every green in regulation en route to the day’s best round, a 67; and Mickelson, who rolled in a 25-footer at No. 18 for his only birdie of the day, giving him a solid 72. They stand at 1-under 139 through two rounds — rarified air at Merion.

“I’m trying to keep a smile on my face,” Horschel said, “and be happy with anything I do.”

How could he not be considering the environs and the characters around him? Merion may be flogging the field, but — even with the second round to be completed Saturday morning — it has created a captivating leader board and a feel that makes predicting the final result a downright guess.

The group at even par includes 21-year-old amateur Cheng-Tsung Pan from the University of Washington, who completed only nine holes before play was halted because of darkness; England’s Luke Donald, a former world No. 1 who shot 72; Donald’s countryman Justin Rose, who hustled to get in his 69 before darkness fell; Ian Poulter, another Englishman, who must play his final four holes Saturday morning; and Steve Stricker, the 46-year-old veteran who now plays a part-time schedule and completed his 69 in Mickelson’s group.

“Tough day,” Stricker said. He could have spoken for the field. The rains that caused so much stop-and-go strife Thursday moved aside, but the wind picked up. The U.S. Golf Association tucked some pins in diabolical spots. Mickelson might have separated himself if not for the tiniest of strokes — a missed two-footer for birdie at 8, another missed two-footer that completed a three-putt bogey at 12 and a sloppy bogey at the little 13th, which played Friday at 123 yards.

“I left a lot of birdie opportunities slide early — in the middle of the round,” Mickelson said. “But I fought hard to stay in there.”

Horschel, precisely the kind of character who pokes his head up at an Open, fought with him. A four-time all-American at Florida, he always has been a feisty sort, so much so that he rankled McIlroy with his exuberant celebrations at the 2007 Walker Cup, the amateur version of the Ryder Cup. His first years as a professional were marred by a wrist injury that limited his play. And when he returned to contend at a tournament two years ago in St. Simons Island, Ga., he melted down in a club-throwing, expletive-laden tantrum.

Now, Horschel is trying desperately to mature. He works with a sports psychologist, Fran Pirozzolo, and he is playing better golf. In April, he closed a series of four straight top-10 finishes with his first PGA Tour win, at New Orleans.

“I’ve acquired some patience — not as much as I wish I had,” Horschel said. “But I just think that the older I get, the more mature I get on the golf course, the more understanding that if I do have a bad stretch of holes — it’s not that I don’t hit the panic button, I just don’t press right away.”

Woods is famous for not pressing the panic button, particularly at this event, which he has won three times. “We have a long way to go, and the conditions aren’t going to get any easier,” he said after his second-round 70.

But it is also difficult to get a read on how much Woods’s ailing left arm — either a painful wrist or elbow — is hindering him because he simply won’t say. On Friday afternoon, when his tee shot at the eighth found the left rough, he hacked the ball out and grimaced before cursing.

Woods said Friday he suffered the injury last month at the Players Championship, which he won. But he wouldn’t say when (other than “one of the rounds”) or how (other than “playing golf”).

What was clear, though, was even at 3 over, he’s still very much in it — four back, tied for 17th, right with McIlroy, managing exhaustion and expectation.

“If you’re a couple over,” McIlroy said, “where you feel like you should be 2 or 3 under par, but it’s not just you that’s struggling out there. It’s everyone else.”

The characters, prominent and obscure, are in place. The course, beautiful and devilish, is a worthy stage. The U.S. Open might as well begin.