Alker takes home US$335,000 ($580,000) for the win increasing his season prize money to US$3.25m ($5.5m).
It was a birdie at the par-4 10th hole, Alker said, that actually put him in a frame of mind to win. Kelly, playing in the same group, made bogey on the hole, dropping into a tie.
"So that kind of changed my mindset a little bit and then, you know, there's a couple of tough holes before we start on the shorter holes," Alker said. "So just get through those and then, you know, try and sprint to the finish."
Drawing even helped him put going 1 over on the last three holes on the front nine behind him.
"Maybe just kind of freed me up a little bit, you know, just kind of relaxed a little bit because I just had the struggles on kind of seven, eight, nine," Alker said. "It just didn't wasn't happening and then hit a quality shot on 10 and to be tied, I wasn't chasing anymore."
Kelly, who led after each of the first two rounds on the James River Course at the Country Club of Virginia, had the lead until the par-4 15th hole, when a slight drizzle and temperatures around 60 changed the conditions and his birdie try stopped on the edge of the cup. Playing partners Alker and Padraig Harrington made their putts to pull even at 12 under, and moments later, in the grouping ahead, Choi also got to 12 under.
"I'm not very strong in those conditions right now," Kelly said. "And, you know, he played pretty well in those conditions. Could have played better. But, you know, if the weather would have stayed the same, who knows what would have happened?"
Alker said it reminded him of home, which was just fine.
"It was kind of like New Zealand weather. Overcast, a little bit of drizzle, and we're just waiting for the fog to come, and I would have felt right at home," he said.
Alker added birdies on the par-5 16th and the par-3 17th, then closed with a tap-in par on the par-5 18th, raising his arms in triumph as it fell, making Choi's closing birdie good enough only for a runner-up finish.
Kelly, who closed with an even-par 72, Harrington (69) and Doug Barron (69) shared third, two shots behind, with Ernie Els (67) and Brian Gay (69) sharing sixth. Els was one shot back after a birdie at the par-4 11th, but finished with seven consecutive pars.