"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."

The 17th was crucial for Mickelson, who stood on the tee box one shot behind. He selected a 4-iron and couldn't have hit it any better.

"I just stood and admired it," Mickelson said. "It was one of the best shots I've ever hit. I mean, it just was right down the center of the green and I was hoping it would kind of get the right bounces. It left me a beautiful uphill putt that I could be aggressive with and I made it. That was fun to do that because that's just not a hole you expect to get one back."

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. Of those 10 players within five shots of the lead, Schwartzel is the only one with experience in winning a major championship. The challenge, however, hasn't changed from the opening tee shot on Thursday.

It's not any player. It's 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."