When the final groups finished the 15th hole Saturday evening at Olympic Club, they walked back down a hill, through a chute lined by fans, and took a right turn. Then they walked, and kept walking. The 15th, on Saturday, played at 108 yards, a benign little par 3. The 16th, on the same day, had the tee box pressed all the way to a cart path. Much further, and it would have sat in Lake Merced. Take the measure, and consider it: 671 yards, the longest hole in U.S. Open history.

“It’s a case,” said Phil Mickelson, “where longer is not better.”

In any of the 112 U.S. Opens, there has been room for quibbles about the course and the way the U.S. Golf Association sets it up. At Olympic, which held up brilliantly the first two rounds, savaging the field, those quibbles have been few.

But the 16th is uncharted territory, and it will come in a spot in Sunday’s round when the tournament will be decided. Two potential birdies, at the 522-yard par-5 17th and the 344-yard 18th, lie ahead. But what to make of the half mile of golf hole, No. 16.

“If you hit two good shots into 16,” Tiger Woods said before the tournament began, “you’re going to have a wedge in there, which you should make birdie.”

The problem, though, is what players have to hit in order to get to that wedge. Bubba Watson, one of the game’s longest hitters, played a Tuesday practice round from the back tee. He hit his pink-shafted driver to start. He found his ball in the fairway, and crushed another driver, this one off the deck. The result: He still had 60 yards left to the pin.

“You can’t reach that hole in two from the forward tee,” Watson said. “I don’t know why it needs to be 670 with the deepest rough of the golf course. There’s going to be people that don’t get there in three because they hit it in the rough and the lie is bad.”

That, in fact, was what USGA Executive Director Mike Davis, the man who has made a reputation setting up U.S. Open courses, wanted.

“I know that there’s a mentality that every par 5 ought to be reachable in two,” Davis said. “But we wanted . . . to make it a true three-shotter, where if you miss one shot you might not be able to catch up on it.”

Mission accomplished. Mickelson predicted before the tournament that 16 would play harder than any hole at Olympic, which, for a U.S. Open course, is relatively short at 7,170 yards. He was almost correct. In Thursday’s first round, the 156-player field averaged 5.564 strokes at 16 — more than half a stroke over par, making it the second-most difficult hole for the day. (On Friday, when the tees were moved forward, it was only the fifth-hardest.)

This was all possible because Olympic put in a new tee box for this Open – a strategy employed last year at Congressional Country Club in Bethesda, where holes such as 9 and 18 were lengthened with new tee boxes. Here, it gave Davis the opportunity to push the limit further back. Previously, the longest hole at the Open was the 12th at Oakmont (Pa.) Country Club, which played at 667 yards in 2007.

The result, as the Open turns to its final round, is that what most players think of as an opportunity — a par 5 — might be a significant challenge.

The hole doglegs sharply to the left, and trees overhang the left side off the tee, making the line very tight.

“Awkward,” Padraig Harrington said. Thursday, Davis Love III hit a perfect driver, a perfect 2-iron, and still had 9-iron into a difficult green complex.

“I bet he would say after two solid shots that’s probably the longest third shot he’s had into a par 5 in quite some time,” said David Toms, who played with Love.

So, then, Mickelson’s problem with it:

“I think great holes provide different strategy and different options,” he said. “With the tee back on 16, it eliminates any options. There’s only one way to play it. . . . But as a player, you have to think about how to play it most effectively.”

Asked if he thought it unfair, Mickelson said, “I would never say it’s unfair. I just wouldn’t say it’s a good hole.”

As if to prove his point, Mickelson made bogeys at 16 in each of his first two rounds. And on Sunday, if someone can’t execute those three solid shots into the green, he might be left with a bogey that goes a long way toward deciding the tournament.