Mahan let his spectacular back nine filled with four birdies go to waste with a bogey-bogey finish for a 69. He will be in the final group for the first time in a major with Mickelson, whom he considers a close friend.
"It's going to be a very, very exciting finish," Mahan said, "because I don't think any lead is safe."
Stricker made a 10-foot par putt on the 18th hole to complete a 70 and perhaps the steadiest round of the day. His only mistake in a round that lasted 5½ hours under sunshine was a tee shot into the water on the par-3 ninth for a double bogey. At 46, Stricker can become the oldest U.S. Open champion.
"I've got to play smart golf ... not make any mistakes," he said. "I think that's the biggest thing. And it's a course where it's tough to come back."
Billy Horschel, tied with Mickelson at the start of the third round, kept his emotions in check and shot 72. He was two shots behind, along with Donald and Rose
The third round featured so much movement, and so many wild swings, that seven players had a share of the lead at some point. Even though USGA executive director Mike Davis said the course was set up to allow for good scores, this was more about hanging on for dear life.
There was no faking it Saturday afternoon.
Thirty players were separated by only five shots at the start of the third round. By the end of the day, there were just 10 players separated by five shots, including amateur Michael Kim. He was tied for third until losing four shots on the last three holes.
For all the talk about Merion being just a short course, the final two holes were beastly — 253 yards for a par 3 surrounded by deep bunkers and framed by the Scottish broom grass, and then a 530-yard closing hole up the hill, deep rough on both sides with bogeys or worse waiting for a single missed shot.
Stricker, remarkably, played bogey-free on the back nine. Horschel, striving for perfection at a championship that doesn't allow for it, dropped only one shot.
"Seventeen and 18, you've got to buckle up and hit good shots," Horschel said. "So I think tomorrow, with the pressure being on, those holes will stick out even more."
Mickelson chose not to carry a driver, and he had to be flawless again on the long closing hole. He swung the 3-wood with confidence throughout the back nine and drilled another. With some 250 yards left, another fairway metal took him just over the green. His chip came out some 10 feet short and he missed the par putt to end a streak of 12 holes without a bogey.
But he still had the lead. It was the first time only one player remained under par through 54 holes at the U.S. Open since 2007 at Oakmont, when there was none. Mickelson was tied for the 54-hole lead at Winged Foot in 2006, where he lost a one-shot lead on the final hole by making double bogey.
Of his five runner-up finishes, that one stung the most.
But he's back for another try to win his national championship. The challenge has never been more severe — not from any player, but from Merion.
Mickelson has one piece of history working against him. In the four previous U.S. Opens at this classic course, no one with the lead going into the final round has ever gone on to win.
"I love being in the thick of it," Mickelson said. "I've had opportunities in years past, and it has been so fun, even though it's been heart-breaking to come so close a number of times and let it slide. But I feel better equipped than I have ever felt heading into the final round of a U.S. Open."