Mickelson, Horschel still on top at Merion

The second round of the U.S. Open concluded Saturday morning with overnight leaders Phil Mickelson and Billy Horschel retaining their one-shot advantage at Merion Golf Club.

On Friday, Mickelson drained a lengthy birdie putt at the 18th shortly after the horn signaled a suspension due to darkness. That gain wrapped up a 2-over- par 72 for the five-time U.S. Open runner-up and squared him with Horschel, who fired a 67 in round two and finished 36 holes at 1-under-par 139.

Luke Donald (72), Justin Rose (69) and Steve Stricker (69) finished at even- par 140. Ian Poulter and amateur Cheng-Tsung Pan were also at even-par when darkness descended, with four and nine holes remaining, respectively. Upon the restart, both players fell down the leaderboard.

Poulter stumbled to bogeys at Nos. 16 and 18 Saturday morning to wrap up a second-round 71 and drop into a tie for 10th at 2-over, while Pan slipped to a 72 and a tie for 20th at 4-over, leaving Donald, Rose and Stricker to share third place.

John Senden (71), Nicolas Colsaerts (72), Charl Schwartzel (71) and Hunter Mahan (69) are knotted in sixth at 1-over through 36 holes.

Tiger Woods, who is battling an elbow injury, and Rory McIlroy are among those tied at 3-over 143. The world Nos. 1 and 2 were paired together over the tournament's opening 36 holes, posting matching rounds of 73-70.

They finished both rounds on Friday as the 113th U.S. Open continues to catch up after a pair of weather delays halted Thursday's action.

The cut line fell at 8-over on Saturday, with a total of 73 players making it through to the third round. Those players will tee off in threesomes beginning at 12:15 p.m. ET, off split tees (Nos. 1 and 11).

Notables who failed to make the cut included Graeme McDowell, Keegan Bradley, Matteo Manassero, Zach Johnson, Boo Weekley, David Toms, Darren Clarke, Angel Cabrera and local favorite Jim Furyk.

NOTES: Horschel hit all 18 greens in regulation during his second round ... Horschel's only other major championship was the 2006 U.S. Open at Winged Foot, where Mickelson lost the lead on the 72nd hole.