Donald dropped a 30-foot birdie putt at his second hole of the round, the par-4 12th, and followed with a chip-in from a greenside bunker at the par-3 13th to claim the lead alone but stumbled with a bogey at the par-4 15th.
That left Donald at three-under overall but having twice taken the measure of Merion’s hardest-playing stretch of holes.
“Those last five holes are very tough,” Donald said. “Those holes were playing long.”
Mickelson, who opened with a 67 on Thursday, awaited a late afternoon tee time with Australian Mathew Goggin third on three-under after three holes of the second round. Scotsman Russell Knox was on the course level at one-under with Belgium’s Nicolas Colsaerts.
In the storm-interrupted first round, Donald entered those holes on a run of three birdies in a row that ended when darkness fell on Thursday after storm delays of more than four hours.
On yesterday morning, Donald took bogeys at 16 and 18, unable to accurately judge the speed of the greens on his first trip through the key stretch on his way to an opening two-under par 68.
“I misread both putts,” Donald said. “I thought they were slightly right to left and they broke well left. I thought I had that one at 18.”
Only five players broke par in the first round on a 6,996-yard course that swatted aside a host of failed challengers, including World No 1 Tiger Woods.