ATLANTA, September 14, 2014 (AFP) - Horschel, who won last week's BMW Championship at Denver after losing the week before only on an errant approach at the final hole, fired his 12th consecutive round in the 60s to finish 72 holes on 11-under par 269.
World number one McIlroy and seventh-ranked Furyk, who has not won since the 2010 Tour Championship, shared second on 272 with England's Justin Rose, American Chris Kirk and Australian Jason Day another stroke adrift.
Expecting the birth of a baby daughter in two weeks, Horschel took the FedEx Cup playoff bonus of $10 million (7.7 million euros) and the $1.4 million (1.08 million euros) top prize for winning the tournament.
Horschel, 27, who entered the four-week playoff run only 60th in points, won more money in one day than he had since turning professional five years ago.
"It's unbelievable," Horschel said. "I was sort of looking toward the start of the 2014-15 season.
"At the same time I knew my game was in good shape and I needed to get out of my own way. I was able to show my game was in good shape these past two weeks."
Horschel credited a putting tip from coach Todd Anderson three weeks ago with sparking the run to glory.
"It was off to the races from there," he said.
McIlroy fired a one-over par 71, stumbling back with three bogeys in a row starting at the ninth. He ran off three birdies in a row starting at the 15th hole but it was too little and too late.
The Northern Irishman praised Horschel but said he was tired after a four-week playoff run and ready for a long rest before the Ryder Cup begins in 12 days.
"I think it was just a week too far," McIlroy said. "I was making mistakes out there that I don't usually make and it got away from me.
"I don't want to see my golf clubs until the Ryder Cup. I want to take a week off and recharge."
McIlroy and Horschel, playing together in the final duo, began the day level atop the leaderboard at nine-under and each birdied the fourth hole to stay deadlocked in the lead.
Horschel and McIlroy had a rivalry dating to their Walker Cup amateur days, when McIlroy took issue with Horschel's vocal style about his own shotmaking during a match.
Horschel sank a 19-foot birdie putt at the fifth to take the lead alone and McIlroy found water off the tee with a 5-iron at the sixth on his way to a double bogey.
It was a similar situation to 2012, when McIlroy hit a 5-iron in the water on the same hole and took double bogey in the final round on his way to missing out on the playoff crown.
- Late Furyk charge fails -
McIlroy took bogeys at the ninth, 10th and 11th holes to stumble out of contention while Furyk made a run for the top with birdies at the second, third and ninth holes to move one back of Horschel and stayed there after both Americans began the back nine with bogeys.
Furyk birdied the par-5 15th, tapping in after leaving a 56-foot eagle putt inches short, to match Horschel for the lead at 10-under.
But Horschel answered with a five-foot birdie putt of his own at 15 to reclaim the lead with three holes to play.
Horschel found the right rough off the 16th tee but rolled in a clutch 30-foot par putt to keep his one-shot lead over Furyk.
Furyk closed with back-to-back bogeys to stumble back, leaving Horschel to par the last two holes for the triumph.
"It was nice knowing after I hit my tee shot on the green at 18 that is was pretty much sewn up," Horschel said.
Scores after Sunday's final round of the US PGA Tour Championship (par 70, USA unless noted):
269 - Billy Horschel 66-66-69-68
272 - Jim Furyk 67-69-67-69, Rory McIlroy (NIR) 69-65-67-71
273 - Chris Kirk 66-68-71-68, Jason Day (AUS) 67-67-70-69, Justin Rose (GBR) 72-66-66-69
274 - Ryan Palmer 69-67-69-69
275 - Rickie Fowler 69-68-67-71
276 - Gary Woodland 71-75-63-67, Sergio Garcia (ESP) 69-71-70-66, Adam Scott (AUS) 69-72-65-70
277 - Russell Henley 70-68-67-72
278 - Matt Kuchar 68-71-69-70
280 - Bubba Watson 67-73-67-73
281 - Cameron Tringale 68-68-74-71
282 - Bill Haas 68-71-73-70
283 - Brendon Todd 70-75-72-66, Jimmy Walker 73-69-69-72
284 - Patrick Reed 67-74-74-69, Kevin Na 70-66-75-73
285 - Zach Johnson 68-74-72-71
286 - Hideki Matsuyama (JPN) 71-71-71-73
288 - Webb Simpson 74-72-72-70, Hunter Mahan 74-72-71-71, Martin Kaymer (GER) 73-69-73-73
290 - John Senden (AUS) 72-75-69-74
292 - Jordan Spieth 71-70-80-71, Morgan Hoffmann 70-73-73-76
300 - Geoff Ogilvy (AUS) 77-77-73-73