Jason Dufner's 63 broke the course record at Oak Hill held by Ben Hogan, Curtis Strange and Webb Simpson, who shot 64 about five hours earlier.
Mickelson shot a 5-under 66 Sunday to match the best round of the tournament and win his first claret jug.
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A day after carding a triple bogey, Bill Haas avoided trouble Sunday at the AT&T National and claimed his first victory of the season.
Bill Haas perseveres after a triple bogey and puts himself in contention to claim his fifth career PGA Tour victory at the AT&T National.