Horschel and Mickelson swapped first- and second-round scores. Horschel produced the three-under 67 Mickelson shot on Thursday, while Mickelson picked up Horschel's 72 from Round 1.
At 9:05 p.m. ET with a few late starters still on the course, 12 men are separated by just two strokes.
Tiger Woods is battling an injury, per Brendan Prunty of the New Jersey-Star Ledger, but he managed an even-par 70 to stay at three-over overall. Woods is just one of the golfers still in striking distance.
In addition to bogeying the first hole of the day, Mickelson also produced back-to-back bogeys on the 12th and 13th hole. During the front nine, he missed several opportunities for birdies.
Because this is the U.S. Open, pars certainly have a greater value. Still, it is hard not to wonder just how good his day could have been with a few conversions on short putts.
Besides Mickelson and Horschel: Luke Donald, Steve Stricker, Justin Rose, Ian Poulter and Cheng Tsung Pan are all just one shot off the lead at the time of this update. All five men will be factors this weekend.
Although Tiger Woods isn't at his best physically, he's still just four shots behind the leaders. Woods' grit and the course difficulty could create an opportunity for him to climb.