Last Updated: 15/06/13 4:31pm
Phil Mickelson: Finished on a high to retain a share of the lead
Phil Mickelson clung onto a share of the lead as the brilliance of Billy Horschel lit up the second day of the 113th US Open.
The adverse weather that blighted the first day finally relented and allowed a full day's play, and Mickelson picked up his first birdie of the day at the 18th to join Horschel in the clubhouse at one under.
Luke Donald, Justin Rose and Ian Poulter headed a strong English challenge, while Tiger Woods and Rory McIlroy both carded second-round 70s to lie just four off the lead.
US unless stated
-1 B Horschel
-1 P Mickelson
E - L Donald (Eng)
E - S Stricker
E - J Rose (Eng)
E - I Poulter (14 holes)
Click here for full collated scores
The brutal Merion layout took its toll on most of the field, but Horschel put together a remarkable three-under 67 to take over the clubhouse lead from Donald.
The American, who lifted his maiden PGA Tour title in New Orleans last month, hit all 18 greens in regulation and carded four birdies against one three-putt bogey at the 13th.
Horschel birdied the tough 18th to get under the card ahead of Donald, who set the early target at level par despite dropping five shots in six holes on the front nine.
The Englishman bogeyed two of the last three as he completed an opening 68, but he got both shots back early in his second round before making a mistake at the 15th - his fifth.
Donald's six at the second was offset by a birdie at the next, but he bogeyed the next four holes before a stunning tee shot to the ninth helped get him back to level par.
He was joined by compatriot Rose, who birdied the first two holes and atoned for an error at the sixth with another pure iron approach to the eighth to set up another birdie.
Rose slipped back to even at the 15th, but a delightful up and down for par at the last capped a creditable 69 - only the third sub-par score of the completed second rounds so far.
Mickelson's opening 67 earned him the first-round lead, but he started his second effort with a three-putt bogey before grinding out 10 consecutive pars.
His run ended with bogeys at 12 and 13, but he nailed a 20-foot birdie putt on the last to salvage a 72, while playing partner Steve Stricker is just one off the lead after he returned a three-birdie 69.
Poulter birdied the eighth and 13th to get back to level par, and he still had four holes of his second round remaining when play was suspended due to darkness with 68 players still on the course.
Fearless amateur Cheng-Tsung Pan was also at level par after nine flawless holes featuring two birdies, while Woods and McIlroy scrambled hard to remain in contention.
Both completed opening three-over 73s in the morning, and Woods twice managed to claw his way back to one over before dropping shots at 14 and 18.
McIlroy stayed hot on the heels of his playing partner after a battling four-birdie 70, raising the prospect of the world's top two being in the same group again in round three.
And as the shadows lengthened the cut mark continued to drop, with those on +7 almost certain to be around for the weekend, that group including Lee Westwood (77) and Adam Scott (75).
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