Hideki Matsuyama stormed to a remarkable sixth victory in his last 20 events as Rory McIlroy's bid for a second WGC-Bridgestone Invitational title petered out in Akron.
Matsuyama fired an eagle and seven birdies in a brilliant closing 61 at Firestone Country Club to finish 16 under par, five shots ahead of two-time major winner Zach Johnson, with Charley Hoffman a shot further back in third.
McIlroy finished fifth alongside Paul Casey, Russell Knox and Adam Hadwin, nine shots adrift of the winner.
He began the day three shots off the pace but made the ideal start with three birdies in the first six holes to trail Matsuyama by one.
However, after finding rough off the ninth tee and coming up short of the green, the four-time major winner hit a clumsy pitch which ran through the green and failed to get up and down to save par.
After missing from five feet for birdie on the 11th, McIlroy's slim chances disappeared with bogeys on the 14th and 15th, although he at least ended on a high note with a birdie on the last.
"I think it's been a good week, obviously wanting to play well this week but with one eye on next week."
McIlroy's victory at Firestone in 2014 was sandwiched between major titles in the Open Championship and US PGA, but the 28-year-old has failed to win a major since and split from his long-term caddie JP Fitzgerald after finishing fourth at Royal Birkdale last month.
"I think it's been a good week, obviously wanting to play well this week but with one eye on next week," McIlroy told Sky Sports.
"I think it was a good prep week; I never want to treat this week like that because it's such a big tournament in itself, but I thought I saw a lot of good stuff to take with me to Quail Hollow.
"I felt like my wedge play got better as the week went on, my iron play got a bit better so those are the two things. If I can sharpen that up and be really good with that part of my game I feel like I have a really good chance."
Matsuyama's bogey-free 61 equalled the course record held by Jose Maria Olazabal, Tiger Woods and Sergio Garcia, but was the first to be recorded in the fourth round and the lowest final round in any World Golf Championship victory.
Another birdie on the ninth took Matsuyama two clear for the first time and although Hoffman and Johnson both closed to within a shot, four more birdies on the back nine - including at each of the last three holes - secured a commanding victory.
Open champion Jordan Spieth needs to win the US PGA to complete a career grand slam, while McIlroy requires a victory in the Masters to join Gene Sarazen, Ben Hogan, Gary Player, Jack Nicklaus and Tiger Woods in having won all four of the game's biggest events.
