Mickelson drained the birdie putt, and despite a bogey on 18, posted an even-par 70 to retain the lead. He heads into his final round at the Open with a 54-hole lead for the second time, but could this finally be the time that Mickelson, who turns 43 on Father's Day, gets over the hump?
"It's got the makings to be something special," Mickelson conceded, "but I still have to go out and perform and play some of my best golf."
For nine holes, Mickelson struggled to get his game going. But he birdied 10 and 11 to get back to even-par on his third round, made par on the next five holes, and came to 17 a stroke behind his playing partner, Luke Donald.
"On the tee I'm just thinking 3," Mickelson said. "I just want to hit the green and make par and see if I can make a putt. . . . It was just right down the center of the green, and I was hoping it would get the right bounces and so forth. And it did. It left me a beautiful putt that I could be aggressive with, and I made it."
It ended up being a 2-stroke swing after Donald bogeyed the hole. The Brit would also double-bogey 18 to finish at 71, leaving him 2 back.
Mickelson will be paired with American Hunter Mahan, who is a stroke back at even-par - along with Steve Stricker and Charl Schwartzel - after shooting a 69. Mickelson has been in the final pairing of a U.S. Open on three previous occasions and has finished runner-up five times.
"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," Mickelson said. "But I feel better equipped than I ever felt heading into the final round of a U.S. Open."
Mickelson said that his ball-striking has been as good as it's ever been, and the numbers back him up. He's hit 30 of 42 fairways (71.4 percent) and 39 of 54 greens (72.2) through three rounds. He also said that his putting has been as good as it's been in years, although he's been up-and-down in that department on Merion's tricky greens.
But a day after needing 33 putts to get around, Mickelson finished with 30 and made some smooth strokes along the way. The most important one may have come at No. 6. He had just bogeyed two of the three previous holes and needed three shots to get to 15 feet above the pin at the par 4.
Mickelson made the downhill putt, carded pars on the next three holes, and then took advantage of Merion's short par 4s at 10 and 11.
"I might be 1- or 2-over in the round like I was, but if I can make a couple of more pars so I get to some birdie holes, I have a chance to get those back," he said. "And that's what I love about Merion - the discrepancy between the really hard holes and the potential birdie holes."
Perhaps more than anything, Mickelson has a 1-stroke lead because of how he's played the really hard holes, especially Nos. 14-18. He's 1-under on those holes through three rounds and will need to master the quarry holes (Nos. 16-18) if he wants to claim his first national championship.
"Those are difficult holes," he said. "You've got to make up ground early. I got off to a poor start yesterday and today."
The Merion gallery has embraced Mickelson, as New York crowds did for Opens at Shinnecock Hills in 2004 and at Bethpage Black in 2002 and 2009. He, of course, finished second in each of those tournaments. Could Philadelphia, fittingly, get Phil over the hump?
Asked if there was a sports team that had finally won a title after coming up short many times that he could draw inspiration from, Mickelson said: "The Eagles have come close a number of times in the Super Bowl, but I don't know. I don't want to go there."
He shouldn't have, considering the Eagles have never actually won a Super Bowl.
Win or lose, Mickelson's quest for his first U.S. Open will be great theater.
Contact Jeff McLane at firstname.lastname@example.org. Follow on Twitter @Jeff_McLane.