Mickelson's 3-under 67 remained the standard when the first round was completed Friday morning, and the nasty rough and hard-to-read greens at Merion Golf Club took an even bigger bite once the second round was under way.
Besides Lefty, the only player under par at one point was Belgium's Nicolas Colsaerts, who, like Mickelson, had a late tee time after shooting 69 on Thursday. Mickelson 3-putted the first hole of his second round to drop to 2 under, cutting his lead to one stroke.
That must-see group of Tiger Woods, Adam Scott and Rory McIlroy was at times a hard-to-watch bunch - perhaps even for Woods' ski champion girlfriend Lindsey Vonn, who was part of the gallery.
The trio combined to shoot 14 over at the halfway mark. Woods' ailing left elbow flared up again, and he hit a chip from just off the green that traveled barely a foot while making bogey at the par-4 7th.
Woods then dropped his left hand off the club and shook his wrist while putting his tee shot wide of the fairway at No. 8, just as he had done several times on Thursday. The arm clearly bothered him again on the next shot, which he put in the rough near the green. He saved par on the hole and finished a second-round 70 that left him at 3 over for the tournament, still in contention if he can stay healthy.
"It's hard with the wind and the pin locations," Woods said. "They're really tough. ... We didn't think they were going to be as severe as they are."
Woods was tight-lipped about his elbow, saying only that it first bothered him at the Players Championship five weeks ago. Asked what he felt, he answered: "Pain."
Masters champion Scott fell apart quickly, all but quashing any hope for a Grand Slam. He was 3 under when first-round play was suspended Thursday, but he hit a hard-luck Merion shot at No. 12: an approach that landed just short of the pin, spun backward and rolled some 75 feet to the edge of the fairway rough. He also put a tee shot out of bounds at No. 14 to complete a first-round 72, then came back after the short turnaround to post a 76 to sit at 8 over through two rounds.
McIlroy had quite the adventure, putting his drive at No. 4 onto the No. 8 fairway. Once he got back to the correct hole, he put a shot in a bunker and bogeyed the par 5. His second-round 70 left him at 3 over.
"I'm very happy. Right in there for the weekend," McIlroy said. "I don't think I'll be too far away by the end of the day."
Then there was Luke Donald, who actually pulled ahead of Mickelson at 4 under with back-to-back birdies, including a chip-in at the par-3 13th during his second round. But Merion took him apart on the front nine when he bogeyed four consecutive holes, turning his number from red to black. His second-round 72 left him at even-par.
"I think everyone thought that as soon as the course got wet it was going to play easy," Donald said between his rounds. "The scores certainly aren't showing that. The tough holes are extremely tough."
Coming into the Open, the question was how Merion would fare against a modern-day championship field. It last hosted this event in 1981, with the thinking that today's golfers had outgrown the course.
Certainly, the 301-yard par-3 10th and 102-yard par-3 13th yielded their share of makeable shots, but pre-tournament concerns about scores in the low 60s seem totally unwarranted.
"This course never lets up," defending champion Webb Simpson said after his opening round 71.
The sun came out Friday after play began in a cool drizzle that was far gentler than the storms that interrupted play twice on Thursday. The fallout from foul weather was a cramped schedule, with some players getting precious little time between rounds on a course that requires long shuttle rides to move them to and fro. It also left players and spectators spackled in mud from their shoes to their pant cuffs.
"I got no choice," Donald said. "Just get out there, grab something quick to eat, hit a few balls and get out there."
Woods completed his first-round of 73, but not before pulling out his driver and putting his first tee shot of the day well right of the fairway into the high grass. On Thursday, he would wince and shake his left arm, particularly his wrist, when playing out of the rough, but there was no obvious sign of discomfort Friday when he played his second shot at the long par-4 No. 12.
There wasn't much else good about the shot, which landed in lesser rough about 100 yards from the hole. Woods then got the ball on the green, but he missed a downhill 5-foot putt and bogeyed the hole to go to 3 over.
Woods birdied the short par-3 13th, but he put his tee shot in the rough near the edge of a steep bunker at the par-3 17th and dropped another shot.
After the round, Woods wasn't in the mood to say much about whatever ailed him. He said his left arm "didn't feel very good" on "a few shots" Thursday. Asked what he felt, he answered: "Pain."
"But it is what it is," he added. "And you move on and I got to get ready for this next round in a little bit."