But then, this was Mickelson, the king of the dramatic.
He bogeyed the first hole, then two more in succession on the back nine. Suddenly, he was no longer the leader and, for a while, was looking as befuddled as he had looked confident Thursday.
But, in typical fashion, he got back a share of the lead he had built with his three-under 67 Thursday.
Darkness loomed and the USGA sounded the horn to end play at 8:27 Eastern time. The Mickelson threesome that also included Keegan Bradley and Steve Stricker was on No. 18. That meant, because they had started the hole, they had the option to mark their balls and return to finish their second round Saturday at oh-dark-thirty or finish the hole. Easy decision.
They were at the green and Mickelson was 35 feet away. With Stricker and Bradley already in close, Mickelson rolled the long putt dead center into the cup for a birdie to get to one under par. Had the ball not hit the cup, it would still be rolling.
Billy Horschel, who had finished hours before with the best round of the day, a 67, is the other leader. Stricker and Justin Rose, in a group of five just one back at even par, shot 69s.
"Every hole at Merion, you fight for par," Mickelson said.
Or, in the case of most of his colleagues, on this day of USGA-arranged pain, you fight for your egos.