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One of the most interesting storylines possible has emerged at the 2021 PGA Championship as 50-year-old Phil Mickelson holds the 54-hole lead and is looking to become the oldest winner in major championship history. The victory would be Mickelson's sixth major championship and second PGA Championship (2005), but Saturday's back nine proved that no lead is safe on Ocean Course at Kiawah Island. 

Mickelson saw a five-shot lead drop to just one stroke in a hurry, and then Brooks Koepka even tied it at 7 under before a bogey at 18 dropped him back to second entering Sunday's final round in South Carolina. Koepka clearly leads the conversation in terms of Mickelson's top competition, but he's far from the only major winner inside the historical and realistic striking distance for the PGA Championship. 

The record for a 54-hole comeback to win at the PGA Championship is seven strokes, set by John Mahaffey in 1978. A six-shot comeback to win has happened just four times, first by Bob Rosburg in 1959 and again by Lanny Wadkins (1977), Payne Stewart (1989) and Steve Elkington (1995). Recently, we've seen our fair share of winners who either had at least a share of the 54-hole lead or only trailed by a couple strokes. Last year, Collin Morikawa trailed Dustin Johnson by two before shooting 64 on Sunday to win by two. 

So with both recent and all-time PGA Championship history in mind, we've set the limits for potential winners other than Mickelson (-7) at seven golfers. Let's break down who is in that group, and what they need to do  in order to hoist the Wanamaker Trophy early Sunday evening at the Ocean Course.

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Brooks Koepka (-6): The pursuit of a third PGA Championship in a four-year stretch really came into picture on Saturday as Brooks put on a ball-striking bonanza but couldn't close the deal with his putter. Koepka shot a two-under 70 but said "one of the worst putting performances" kept it from being a really special round, noting that his speed was off all round. Statistically, he was about field average with the putter in terms of strokes gained, and he stands alone as the top player in the field in strokes gained from tee to green through three rounds. 

Louis Oosthuizen (-5): Speaking of leaving strokes out on the green, Oosthuizen had one of the 10 worst putting performances in the field on Saturday. He continued to hit it well from tee-to-green, but the 2010 Open Championship winner could not find enough birdies to seize the lead despite hitting 75% of his greens in regulation in the third round. Just two shots back, Oosthuizen is absolutely a contender to win, and he might even be a good bet if you believe the putting woes are temporary and a return to field average putting could bring a low round on Sunday  

Kevin Streelman (-4): With not even a single top-10 finish at a major championship, the 42-year-old Streelman is already in position to have one of his best-ever finishes heading into Sunday. His best PGA Championship finish was a T12 back in 2013, and if he continues to be as steady as he's been through a three rounds, he's likely going to have that first top 10 at a major. But will Streelman win? He's not my favorite bet of the bunch, but he's in the appropriate striking distance to be considered a real contender. 

Rick Gehman and Mark Immelman break down and react to Saturday's third round action at the PGA Championship. Follow & listen to The First Cut on Apple Podcasts and Spotify.

Christian Bezuidenhout (-3): Maybe a little bit of a stretch to think the 26-year-old South African is going to storm back from four strokes down to conquer a leaderboard that includes multiple major winners, especially considering this position has a lot to do with a hot putting performance on Thursday. But Bezuidenhout is playing the best golf of his career right now, and an epic round on Sunday could be enough for his first major win. 

Branden Grace (-3): It was a mixed bag of results for Grace on Saturday, but he avoided some big numbers and closed strong with a birdie on No. 18 to move into a spot four strokes off the lead. We've gotten used to seeing Grace near the lead at a major, but is now the time we see him convert it a win? 

Bryson DeChambeau (-2): A 5-stroke deficit is where you have to draw the line for contenders, and even at 2 under, the only names that are going to jump out as potential comeback kings are proven major winners. DeChambeau fits the bill thanks to his U.S. Open win last year and T4 finish at the last PGA Championship, but he's going to need to go really low in order to make up ground on leaders who are more than comfortable defending their lead on a Sunday afternoon at a major championship. 

Gary Woodland (-2): Like DeChambeau, we can check the boxes for being a prior major winner and one of the top talents in the game right now as reasons to think Woodland can get it done. Three double bogeys on Saturday do not necessarily amp up the confidence in him making a run over the final 18 holes.