Needing a birdie at the par-four last to etch his name into golf's history books, Dufner left his 12-foot uphill putt 18 inches short of the cup, then tapped in for par to complete a seven-under score.
Known for his unflappable and ultra-laidback persona, the 36-year-old became the 24th player to shoot a 63 at a Major on a day when Oak Hill's East Course was ripe for plundering after being rain-softened during the week.
Dufner holed out with a sand wedge from the fairway to eagle the par-four second, then reeled off five birdies over the next 13 holes on the way to a two-shot lead in the year's final Major, at nine-under 131.
"It was a great day for me, a good day to get out there and score," Dufner said after tying the US PGA Championship low for 36 holes, and improving the course record at Oak Hill by a stroke.
"Obviously the rain, the soft golf course made it scoreable but to join history, to shoot a 63 in a Major, is pretty unbelievable, and to be leading the tournament is even better. Hopefully it will propel me to a great weekend."
Asked whether he had been aware of how his own round stacked up to the record books, Dufner replied: "I knew that nobody had shot 62, and the course record (of 64) I had heard on TV earlier in the week.
"So I knew where I stood and you couldn't have a better putt for a chance at history on the last hole but I just didn't quite hit it hard enough."
Dufner, a double winner on the US Tour, was delighted with his first two shots on the final hole as he chased a sixth birdie of the day.
"I hit a great tee shot," he said. "Then I was a little bit in between clubs, in between a five and a six-iron, from 203 yards (for his approach), so I went with a six and left it in a perfect spot.
"The greens had gotten a little bit slower, obviously a little bit bumpier and I probably didn't hit the best putt, probably the worst putt I hit of the day, which is a little disappointing.
"But all in all, it's a 63, and my name is on top of the leaderboard, so that's a great position to be playing from. If I could do it over, I guess I would hit the (birdie) putt harder."
Dufner's closest bid for a Major victory came in the 2011 US PGA Championship at Atlanta Athletic Club where he led by five strokes while playing the fourth-last hole before losing steam.
He ended up losing the title in a playoff with US Tour rookie Keegan Bradley, but believes that experience, and his two victories on the US circuit last year, can only stand him in good stead at Oak Hill this week.
"Atlanta is probably the best I've ever hit it in my career for that specific week," he said.
"I played really flawless there for 68 holes or 69 holes. That's probably the best I've hit it in my career. Today, I scored better.
"I've got more experience now playing in these Major championships, winning a couple of events, being close to winning some more events since Atlanta, so that will always help."
Asked whether he was as laid-back on the inside as his demeanour suggested, Dufner smiled: "I'm probably like everybody else but I can hide it a little bit better.
"Today was a little bit unusual because just of the buzz that was going on with the round that I was producing. Usually get that buzz towards the end of the championship.
"When you're chasing history, it's tough. But in my head, I was just trying to get further and further away from the field, trying to make birdies."
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