He had the option to take out a ball marker, place it on the green and call it a night; he certainly didn't expect to make such a tricky putt at the end of a day when putting had all but unmade him.
In the twilight of the day, nearing the twilight of his career, Mickelson, who turns 43 on Sunday, stepped to his ball and took a good long look at the hole. He let it go.
And then there was a roar in the dark.
Mickelson, the people's choice, was back to 1-under par, good enough for a share of the lead.
It had felt all day that heartbreak was slowly creeping toward Mickelson. He held the lead for hours while he waited for his round to begin, and just about every other golfer in this Open was stumbling, some badly. The course was going to come after him too, the moment he stepped onto it.
Sure enough, Mickelson started his round at nearly 4 p.m. with a bogey. He fought back valiantly with a long string of pars, and then came the inevitable gut-punch: an easy par putt missed on 12, followed by another bogey on 13, the easiest of Merion's 18 holes. The lead was gone. He was back to even par, as if everything he had done over two days was gone.
He trod up 14 slowly, his sure smile melted into a look of concern. His eyes were reddened. His hands were on his hips. His visor was pointed down.
The scene looked like something akin to a slow drowning, just the way other leaders had raced into the red and then wilted into the black. The roar of the early afternoon had given way to worried encouragement from the winnowing gallery. "Do this, Phil!" one yelled. Another lit up a cigarette and folded her arms.
There's always been this balance of wild hope and deep dread with Mickelson. His wins have been epic; his losses have been frightful. Could a man so prone to that one mistake stand up to a course that kills with a thousand cuts? He had never won the U.S. Open, finishing second a record five times. Could he finally win here?
Merion has laid out even the best players. Nearly five hours before Mickelson's final putt fell, three weary champions walked away from their last hole of the day, their faces creased with exhaustion and regret. Tiger Woods, Rory McIlroy, and Adam Scott were all supposed to dazzle; instead the course had left them dazed, Tiger injured.
They cut across the first fairway to the scorer's tent – the only shortcut Merion allowed them all day – and the roar of the crowd that always follows Woods and McIlroy suddenly exploded.
Mickelson was lumbering straight at them, chasing after his first tee shot of the day, grinning like a madman.
The throaty cheers had met and multiplied like two thunderclouds building over the plains, and it was hard not to witness the scene and wonder if the paths of these men – the best golfers of this generation – are bound to cross again. But it was also hard not to wonder if Mickelson's wide smile would last the weekend. Or the day.
"This is the stiffest test of par 3s we've ever faced," Woods said moments later, which is quite the statement considering the two toughest holes on the course are not par 3s.
"It feels like you can shoot 66," said McIlroy, who along with Woods is plus-3, "but where these pins are …"
McIlroy wondered, almost to himself, what the 18th will be like with a weekend of good weather. He suggested tee shots would be impossible to keep on the fairway without some sort of idiotic slice. We've learned a player must stay in the fairway to win, yet what if that's not possible?
As Woods and McIlroy spoke of the par 3s and the nasty finishing hole, Mickelson trudged on, getting closer to that 18th. There was only one player under par for the entire tournament – Billy Horschel at 1-under – and Mickelson would need something outrageous to match that now.
"Throughout every hole at Merion," he would later say, "you fight for par, fight for par …"
Then he arrived at the 18th green even for the tournament, and there was one more fight for birdie. There was one more shot in the dark.
If you believe in history, you can feel it unfolding here. This is the course that bequeathed Bobby Jones, Ben Hogan, and Lee Trevino versus Jack Nicklaus in a playoff. There is a sense that something of great magnitude is getting ready to happen. There is a sense that a legendary name will be called on Sunday night.
Eventually this tournament will belong to history. For now, it belongs to Phil.
More U.S. Open coverage from Yahoo! Sports
• Breaks falling Phil Mickelson's way at Merion
• Merion chews up past major champions
• Billy Horschel sets Merion record, tied for Friday lead
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- Merion Golf Club
- Tiger Woods