* Maiden win for Walker after 188th career start
* Koepka squanders four-shot lead in final round (Adds detail, quotes)
Oct 13 (Reuters) - American Jimmy Walker took advantage of a late collapse by overnight leader Brooks Koepka to win his first PGA Tour title by two shots at the Frys.com Open in San Martin, California on Sunday.
Three strokes off the pace heading into the final round, Walker coolly birdied two of the last six holes to close with a five-under-par 66 on a sun-splashed afternoon at CordeValle Golf Club.
The 34-year-old seized the outright lead for the first time when he two-putted for birdie at the par-five 15th, then made three consecutive pars to post a 17-under total of 267 in the opening event of the PGA Tour's 2013-14 season.
"I think it will sink in after a while," an emotional Walker told Golf Channel after earning a Masters invitation and a two-year exemption with his maiden win on the U.S. circuit following his 188th career start.
"I am still feeling the adrenalin and everything. I feel like a relief, a release kind of, right now but it feels really good. I am pretty excited.
"I was swinging good so I just kept saying, 'Let's just keep hitting good shots, putting good.' Everything felt like it was in control, so I just kept doing that and it worked out."
American Koepka, also seeking a maiden victory on the PGA Tour, briefly led by four shots on the front nine but tumbled backwards with bogeys on four of the last 10 holes on his way to a 72 and a four-way tie for third place.
Former world number one Vijay Singh, bidding for his first win in five years on the PGA Tour where he has triumphed 34 times during a Hall of Fame career, finished alone in second at 15 under after closing with a 68.
For the early part of the afternoon, Koepka appeared to be heading toward a remarkable victory in only his fourth career start on the PGA Tour, stretching his overnight lead to three shots when he sank a 10-foot birdie putt at the par-four second.
Singh put some early heat on the pacesetting American, birdies at the first and third putting the Fijian two strokes off the lead before he stumbled with a three-putt bogey at the seventh.
Moments later, Koepka drained a monster putt from 45 feet to birdie the par-four sixth and briefly move four ahead of the chasing pack.
However, Walker birdied the eighth and ninth to get to 15 under and Koepka then three-putted from the back fringe to bogey the par-five ninth for his lead to be cut to just one.
Koepka also bogeyed the par-three 11th, after missing the green to the left and chipping five feet past the cup, to drop back into a tie for the lead with Walker.
Though Koepka immediately recovered with a birdie at the 12th to regain a one-stroke cushion, Walker joined him at 16 under soon after by rolling in a 30-footer for birdie at the 13th.
Walker grabbed the outright lead with his birdie at the 15th and Koepka, playing one group behind, squandered an opportunity to match him when he missed a six-footer there.
Koepka bogeyed the short 16th after missing the green to the left with a nine-iron and his title hopes disappeared with a bogey at the driveable par-four 17th where he found water off the tee.
"Obviously today wasn't the day I was expecting or anything but it's still a good tournament," said the 23-year-old Koepka who has triumphed four times on the lower-tier European Challenge Tour since turning professional in 2012.
"You've got to take the bright side and move on to next week (for the PGA Tour's Shriners Hospitals for Children Open in Las Vegas)." (Reporting by Mark Lamport-Stokes in Los Angeles; Editing by Julian Linden and Gene Cherry)