Potter wins Greenbrier in playoff
Potter, ranked 218th in the world, erased a four-shot deficit with five holes remaining in regulation, carding a final-round six-under 64 to join Kelly on 16-under 264.
Kelly, who started the day two shots behind overnight leader Webb Simpson, had five birdies in his four-under 66 on the Old White TPC course.
Simpson, who claimed a first major title with his US Open triumph at The Olympic Club in San Francisco last month, surrendered a one-shot lead on the back nine of the final round here for the second straight year.
Four bogeys in five holes from the 12th spelled the end of his challenge and he closed with a 73 that left him tied for seventh.
South Korea's Charlie Wi and rookie Charlie Beljan shared third place on 14-under 266. Wi posted a 65 and Beljan a 67.
Potter became the second rookie to win on the US tour this season.
After capping his fourth round with an eagle and a birdie, he had a chance to win at the second extra hole -- the 17th that he had eagled in regulation.
After coming up short of the green, Potter chipped on to about five feet but missed the birdie putt as Kelly saved par from a greenside bunker.
Returning to the par-three 18th, Potter landed his tee shot four feet from the pin and made the putt for the victory.
He became the sixth first-time winner on the tour this season. He earned a winner's prize of $1,098,000 and jumped from 173rd to 51st in the FedEx Cup playoff standings.
It was quite a turnaround for the 28-year-old left-hander, who had missed five straight cuts coming into the tournaments.
Kelly still notched his best career finish, improving on a prior tie for 37th.