On Sunday, Mahan, whose best Open finish was a sixth-place tie in the rain at Bethpage Black four years ago, will play in the 3:20 p.m. final pairing with leader Phil Mickelson, a man who figures to have the majority of another large and vocal crowd in his corner.
"It's exciting," Mahan said of the prospect. "I feel like my game's been good for a while and I felt like this course suited me pretty well."
Curiously, two of his competitors in a top-of-the-leader-board devoid of previous U.S. Open winners - five-time runner-up Mickelson (209) and Steve Stricker (210) - were among Mahan's most vocal defenders three years ago when he lost the deciding Ryder Cup match to Graeme McDowell on the 18th hole at Celtic Manor.
But friendship and loyalty will mean little the way Merion has been playing this week.
"This course is very penalizing," he said, repeating the week's most popular mantra. "If you hit an errant shot, you're going to have a tough time making par. Even if you hit a good shot, you'll still have a putt that probably is going to break a lot and have difficult speed."
Mahan bogeyed the fifth - a hole that is getting increasingly difficult as the heat and sun harden its sloping fairway and green - and was 1 over at the turn.
He took advantage of the back's soft beginning and birdied holes 10, 12 and 13. Another at 16 briefly gave him the tournament lead at 2 under. And then came bogeys at the brutal 17 and 18.
He wasn't alone in wrestling with those two bearish holes.
Of the 15 golfers at the top of the Open scoreboard Saturday night, only four parred the 530-yard finishing hole. Luke Donald (211) took a 6, Nicholas Colsaerts (215) shot a triple-bogey 7.
"Eighteen is 530 yards. Uphill," Mahan said. "So obviously it's a brutal hole."
He was asked, as so many of the weary and beaten players were, if he thought the U.S. Golf Association's pin placements were too draconian.
"There aren't many [places to put the] pins," he said. "There's like four maybe and on some holes there's only three just because of the speed of these greens and the slope."
Mahan, 31, has occupied a prestige level just below the major winners for the last few years. He was ninth on the money list in 2012 and has been no worse than 16th since 2008. But he's never really come close in a major, never had to grind it out in the kind of Sunday pressure he will experience Sunday.
"I'm a good ball-striker," he said. "I've got to get my short game a little better to save those big ups and downs and make those six- and eight-footers for par. I felt like I did that today. And it feels good to be in the hunt and be in contention."
How it will feel tomorrow at about 7 p.m. is another matter.
"It's going to be a very, very exciting finish because I don't think any lead is safe," he said. "And there's an opportunity to make two or three birdies in there. So it's a great finish."
Contact Frank Fitzpatrick at ffitzpatrick@phillynewscom. Follow @philafitz on Twitter.