No major championship goes according to plan. The unpredictable element of professional golf is what keeps fans watching until the final shot drops.
Thus far, the PGA Championship has lived up to expectations in terms of surprise. The top of the leaderboard is littered with underdogs, while some of the game's greats have fallen far behind.
Through 36 holes of action at Oak Hill, here's a review of some of the biggest head-turning developments.
The four-time major champion won't take part in weekend action after falling three shots shy of the cut (three over). A first-round 74 doomed Ernie Els, although he didn't do himself any favors with a 72 on Friday.
The 2012 British Open winner was competitive during major tournament competition earlier in the year. Els finished fourth at the U.S. Open and tied for 13th at the Masters.
He bows out of the PGA Championship unceremoniously.
If you were asked which player in the field at Oak Hill was primed to tie the major record for lowest-scored round, how many guesses would it take to get to Jason Dufner?
The 36-year-old with two career PGA Tour victories soared to the top of the leaderboard Friday, catching fire en route to a seven-under-par 63. No player in history has ever compiled a better score at a major tournament.
Dufner, who coughed up a five-stroke lead with four holes left at the 2011 PGA Championship before falling to Keegan Bradley in a playoff, enters the weekend in control. He sits at nine under for the tournament, leading a trio of players by two shots.
His amazing Friday afternoon effort quickly dawned on Dufner afterward.
''The history of the game is something dear to my heart,'' he told AP writer Paul Newberry via Yahoo!, ''To be part of history, to be there forever, is a neat accomplishment. I never thought a guy from Cleveland, Ohio, would be able to do the type of things I've been able to do.''
Adam Scott is seven under through two rounds.
The field is largely flourishing through 36 holes. When action resumes Saturday, six players will begin at six under or better (Jason Dufner, Adam Scott, Matt Kuchar, Jim Furyk, Justin Rose and Henrik Stenson). It's a far cry from the U.S. Open and Open Championship, the past two major tournaments.
Every player finished over par at the U.S. Open at Merion, where Rose won at one over. Phil Mickelson was the only competitor to complete four rounds at Muirfield under par during the British Open.
Players are scorching the turf at Oak Hill, where we've already seen four sub-66 scores. If the tournament continues at this pace, Sunday's back nine action will feature multiple players with double-digit under-par scores.
Last Friday, Tiger Woods tied his career-best with a 61 at Firestone. The legend's vintage performance propelled him to an seven-stroke victory and created an elevated level of hype with the PGA Championship approaching.
Fast forward to present day and it appears Woods' five-year major title drought is destined to extend into 2014. He double-bogeyed the final hole of the first round, finishing Thursday one over par.
Woods was equally unimpressive Friday, meandering his way to a second-round 70. He enters the weekend trailing by 10 strokes.
Tiger owns five tournament titles on the PGA 2013 Tour. Once again, he is solidified as the No. 1 player in the Official World Golf Ranking.
Yet, none of that equates to the completion of his comeback. For many, Woods won't truly be "back" until he wins that elusive 15th major championship.
Phil Mickelson is in even worse shape than rival Tiger Woods. Lefty is 11 strokes off the lead after posting a 71 in both the first and second round.
The future Hall of Famer surged on Sunday at Muirfield to seize a three-stroke victory at the Open Championship, but he'll need even more magic to pull this one out with a second PGA Championship win. Since logging back-to-back victories at the Scottish Open and British Open, Mickelson has cooled off considerably.
He tied for 21st at the Bridgestone Invitational, where he finished one over par. Through two rounds at Oak Hill, Mickelson is tied for 50th place, adding another chapter to a career of ebbs and flows.
Before a second-round 76 derailed his contention at Oak Hill, David Hearn rose up the leaderboard with a 66 on Thursday. He missed the cut in eight of 22 PGA Tour starts this season, but six birdies set him up at four under through Round 1.
That effort ultimately ensured Hearn made the cut. The Canadian did not compete at the Masters or Open Championship.
Martin Kaymer hasn't won a PGA Tour event since taking the HSBC Champions event in 2011. A first-round 68 set the tone for an impressive tournament at Oak Hill.
Kaymer capitalized on a strong run Thursday, tallying birdie-eagle-birdie on holes 12 through 14. He enters the weekend at four under, five strokes off the lead and in the thick of things at a major tournament for the first time in over a year.
PGA rookie Jordan Spieth stole headlines last month when he won the John Deere Classic. Just 19 at the time (he's since turned 20), Spieth became the youngest PGA Tour winner in 82 years.
His first full season on the Tour has been a revelation. Aside from the historic win, he owns six top-10 finishes in 2013.
However, the wheels came off the wagon at Oak Hill. Spieth finished Friday at eight over, missing the tournament cut by five shots.
We'll see how the youngster handles his first heavy dose of adversity as a pro.