The third round likely marked the end of Tiger Woods' bid to end a drought in majors that extends back five years. Woods shot a 76 and goes into the final round looking at a 10-stroke deficit.
A four-time major champion, Mickelson has been in this position before but has fallen short, finishing as the Open runner-up five times. However, this is the first time he has held sole possession of the lead entering the final round; he was coleader at Winged Foot with England's Kenneth Ferrie in 2006.
And he said he won't feel the heat of having to win one, even though his chances may be diminishing as he gets older.
"I don't think I feel any more pressure than anybody else who wants to win the tournament, a major championship, the U.S. Open," he said. "But it would certainly mean a lot to me. This is a tournament for years I've had opportunities, I've come close. It would mean a lot [Sunday] if I could play some of my best golf, certainly if I can play the way I have been."
Mickelson showed the patience of a veteran on Saturday, especially on the front nine, where the birdie putts weren't dropping. He was 2 over for the day at the turn and trailed by 3 strokes.
But the round turned at the 10th and 11th holes where he sank birdie putts of four and 15 feet. He topped it off with a 10-footer for a birdie 2 at the 254-yard 17th hole. Even though he finished with a bogey at the brutal 18th, he still had a 1-stroke lead and plenty of optimism.
"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 heartbreaking 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."
Mickelson referred to his ball-striking and putting as better than ever, and said he feels "very comfortable" on a Merion course that has taken a chunk out of many of the world's best players this week.
Only nine players were within 5 shots of Mickelson entering the final round. Other than Mickelson, the other challengers have combined for one major - Schwartzel's win in the 2011 Masters.
Schwartzel gained the lead with a birdie at No. 10 and was joined in first by Donald and, later, Mahan. But Schwartzel and Mahan bogeyed their last two holes, and Donald finished up bogey-double-bogey. Still, Schwartzel and Mahan broke par with 69s, and Donald had a 71.
"Whenever you shoot under par on Saturday at the U.S. Open, you can't be too disappointed," Schwartzel said. "Anything under par is fantastic here."
That will be the goal on Sunday for everyone, particularly those in the top 10. Stricker, at 46 and 0 for 58 in majors, will join Mickelson in receiving sentimental support. Then you have the twentysomethings - Schwartzel (28); Horschel (26); Jason Day (25); and Rickie Fowler (24), who moved up 28 spots in the standings with a 67, the best round of the day.
But all eyes will be on Mickelson. He welcomes the opportunity.
"It's got the makings to be something special," he said, "but I still have to go out and perform and play some of my best golf."
Contact Joe Juliano at firstname.lastname@example.org. Follow on Twitter @JoeJulesInq.