"I think it will sink in after a while," Walker said. "Relief right now. It feels really good. I'm pretty excited."
It was a sinking feeling for Koepka, whose journey around the world in a remarkable year nearly ended with a PGA Tour card.
Koepka had a four-shot lead with 11 holes to play, and the 23-year-old Floridian looked as poised and confident as he had all week at CordeValle. There was a tiny crack when he missed a 3-foot par putt on the ninth hole, and it really caught up with him at the end.
Koepka missed a 6-foot putt on the 15th that would have matched birdies with Walker, who was playing in the group ahead of him. Koepka then missed his tee shot wildly to the left on the 16th hole and had to scramble for bogey. Down to his last shot, he hit into the water on the 297-yard, par-4 17th for another bogey.
"Things weren't going my way," Koepka said. "I didn't make the shots I needed to win. Congrats to Jimmy on that. I'll try to learn from the whole experience."
Vijay Singh closed with a 68 and wound up with the 27th runner-up finish of his Hall of Fame career, and his best result since he sued the PGA Tour in May over its handling of the investigation into Singh admitting he used deer antler spray.
Koepka, who closed with a 72, wound up in a tie for third with Kevin Na, Puerto Rico winner Scott Brown and Hideki Matsuyama, the 21-year-old Japanese star who is playing the PGA Tour for the first time. He was in the Presidents Cup last week.
The Frys.com Open is the season opener in the new PGA Tour schedule, which starts in October instead of January for the first time in history. Instead of a Fall Series event where players mainly were trying to make money to keep their cards, the tournament offered all the perks of any other regular PGA Tour event.