ARDMORE, Pa. -- Phil Mickelson drilled a long birdie on his final hole and Billy Horschel made every green in regulation on Friday to be the only two players below par after a long and trying second day of the U.S. Open.
Nearly half the field did not finish the second round when it was suspended by darkness.
Moments after the horn sounded to stop play, Mickelson opted to finish his round and sank a 20-foot birdie putt for a 2-over 72. That gave him a share of the clubhouse lead at 1-under 139 with Horschel, who made it as easy as possible by hitting every green in regulation for a 67.
Even with the round not finished, it was becoming clear that this U.S. Open might be up for grabs until the very end. Tiger Woods, who grimaced with every shot out of the rough because of pain in his left elbow, was at 3-over 143 and still very much in the game.
"I don't know how anyone is going to separate too far from the field," Mickelson said. "There might be a hot round tomorrow, and they might get a hot round on Sunday, but unlikely to be the same player."
No one was hotter than Horschel, playing in his first U.S. Open since he was a 19-year-old in college.
Nothing is tougher than Merion, the little course in the suburbs of Philadelphia that even in rain-softened conditions is showing plenty of might. And to think there was chatter at the start of the week about the potential for the first 62 in major championship history.
"Perhaps next time you guys will believe when we say it's really not that easy, that it's really not that easy," Geoff Ogilvy said after a 70. That put him at 4-over 144, which gave him and dozens of others a legitimate shot going into the weekend.
Luke Donald (72), Justin Rose (69) and Steve Stricker (69) were at even-par 140.
The surprise were a pair of amateurs -- American Michael Kim and Cheng-Tsung Pan of Taiwan. They were 2 under for their round and among those who didn't finish.
The long day, brought on by storm delays on Thursday, began with cool conditions and patches of light rain that eventually gave way to sunshine. That led players to wonder how much tougher Merion will be once it starts to dry out.
"It's not as easy as people think," defending champion Webb Simpson said after a 75 put him six shots behind the clubhouse lead. "I heard 15, 16 under floating around. And it's going to be a normal U.S. Open winning score."
Horschel hit all 18 greens in regulation, a stellar achievement at a regular tour event, let alone the U.S. Open. That last documentation of someone doing that was Johnny Miller when he closed with a 63 at Oakmont to win in 1973.
David Graham used his putter on every hole -- three from the fringe -- when he shot 67 to win the 1981 U.S. Open at Merion.
"I didn't know I hit every green until I walked off 18," Horschel said. "It's a cool thing. But like I said, it's not the first time I've hit all 18 greens. I've done it plenty of times in my career. It's at a U.S. Open, but the softness of the greens helped that."
Pan played nine holes and was even par, along with Ian Poulter, who was 1 under for his round through 14 holes. John Senden of Australia had a 71 and Nicolas Colsaerts of Belgium shot 72 to finish at 1-over 141.
Mickelson, equipped with a full night of rest after his cross-country trip Wednesday from his daughter's eighth-grade graduation in San Diego, kept battling until ending on a sweet note.
With that birdie putt on his final hole, Mickelson was under par through 36 holes for the seventh time in the U.S. Open. The previous six times, he was a threat to win on Sunday. Mickelson has five silver medals as a runner-up, and all he wants is another chance.
"I just like being in the mix," he said. "It's fun having a chance heading into the weekend. The way I have control off the tee and as good as the putter is -- even though it didn't show today -- I'm very excited about the opportunity this weekend."
Horschel's only bogey was on the 13th hole, the short par 3 and the easiest at Merion.
"I was not in the zone, trust me," Horschel said. "This golf course, even though it's soft, is still a tough golf course. I know what `in the zone' is for me. I don't get nervous, I just see the shot and go. And I saw the shot and I went with it, but I was still nervous with a lot of them. Your misses here can be bad if you miss in the wrong spot."
Woods, for all his travails, was only four shots behind. So was Rory McIlroy, who also had a 70.
"It tests every aspect of your game," McIlroy said. "There were people talking about 62s and 63s at the start of the week and I never saw that at all. Something very little under par is going to win this week. If not that, around even par."
Everything looked like a grind for Woods, who said he first hurt his left arm at The Players Championship a month ago. He dangled the arm and occasionally grimaced with shots out of the rough.
Trying to end five years without a major, Woods was keeping his head down.
"Just keep grinding," he said. "You just don't ever know what the winning score is going to be. You don't know if the guys are going to come back. We have a long way to go, and these conditions aren't going to get any easier."