That round meant Atwal ended at three-over 283.
Atwal opened the final day with three birdies in a row and added a fourth on the fifth. Then after a series of pars, he turned in four-under. But on the back nine, he double bogeyed the 10th and the 16th and finished bogey-bogey on 17th and 18th.
Berger had never been in a final pairing until this week and apparently Mickelson needled him in the players' dining area about having won the Rookie of the Year award without winning while they waited out the lightning.
Then, Mickelson, came within a stroke on the back nine. Berger responded with birdies on three of his next four holes, highlighted by a 32-foot putt on the par-3 14th that pushed his lead to 13 under.
Mickelson and Stricker shot 67, and Koepka had a 66. Dustin Johnson had a 63, setting the back-nine record with a 29, to finish fifth at 9 under.
Now Berger heads to the US Open in Oakmont and will try to become the first man to follow up a TOUR title by winning the US Open.