World number one Rory McIlroy owns a one-stroke lead over Austrian Bernd Wiesberger, a two-stroke edge on Fowler and a three-shot advantage over Mickelson at rain-soaked Valhalla.
But with the wet course receptive for aggressive shotmaking and a host of rivals hungry for their first major title within reach of McIlroy, the stage is set for a final round that could resemble a Kentucky Derby backstretch run from nearby iconic horse racing venue Churchill Downs.
"You've got to go out and make birdies. You just have to get a hot hand."
The 44-year-old US left-hander, ranked 13th, has struggled most of the year, his lone top-10 showing a runner-up finish at Abu Dhabi.
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But last Sunday, Mickelson fired a stunning 10-under par 62 at the World Golf Championships event in Akron, Ohio, and found the magic in his swing he had been seeking all season.
Now Mickelson has one round to turn back the clock and try to bring the form that won him last year's British Open, the 2005 PGA Championship and three Masters green jackets.