CHERRY HILLS VILLAGE, September 7, 2014 (AFP) - Six days after squandering a chance to win in Boston by plunking an approach into rough terrain on the last hole, Horschel kept his nerve down the stretch as rivals faded to win the $1.4 million top prize.
"It feels so good. It means a lot," Horschel said. "This feels good."
Horschel fired a one-under-par 69 to finish on 14-under 266 and defeat reigning Masters champion Watson by two shots at Cherry Hills in Colorado.
It is just his second PGA title, the first coming last year at New Orleans.
"After last week, everybody on Twitter wanted to call me a choker," Horschel said. "I didn't choke. I made a bad swing at the wrong time. You don't choke when you are coming from behind.
"It meant a lot that I could win this one."
Horschel parred all the way through the back nine, knocking in a crucial seven-foot par putt at 15 and a vital eight-foot par putt at 16 as challengers faded late.
"You're just grinding out there and I made some big putts," Horschel said. "I can trust in my putter not to let me down from inside of 10 feet."
Horschel's victory vaulted him into second place in the US PGA playoff points standings, nine points behind leader Chris Kirk and among those who can take the playoff crown by winning next week's season-ending Tour Championship in Atlanta.
World number one Rory McIlroy, who finished fourth in points, joins Americans Kirk, Horschel, Watson and Hunter Mahan in being able to claim the playoff title with victory in the 29-man field next week at East Lake.
American Morgan Hoffmann fired a 63 to finish third on 269 and play his way into the Tour Championship, thanks to four birdies in a row starting at the par-5 11th and an eagle at the par-5 17th.
Spain's Garcia joined Jim Furyk, Rickie Fowler and Ryan Palmer sharing fourth on 271, one stroke ahead of McIlroy, Australian Adam Scott and American Jordan Spieth.
Palmer made the first charge at Horschel, matching him for the lead with a birdie at 11, only to bogey 12 and double bogey 13 to fall back.
Garcia birdied the par-3 15th to pull within a stroke of Horschel, but hit his approach at 17 over the green, then chipped across the green and into the water on his way to a triple bogey that ended a run of 20 holes without going over par to doom his title bid.
Watson had the final push with birdies at 16 and 17, but he missed a 25-foot birdie bid at 18 and Horschel, who raced off the 18th green for a quick bathroom break while his playing partners hit their third shots, had no problem finishing off the triumph.
Leading scores after Sunday's final round of the US PGA's BMW Championship (par 70, USA unless noted):
266 - Billy Horschel 68-66-63-69
268 - Bubba Watson 70-66-66-66
269 - Morgan Hoffmann 72-72-62-63
271 - Jim Furyk 70-68-67-66, Rickie Fowler 71-66-66-68, Sergio Garcia (ESP) 68-64-72-67, Ryan Palmer 69-64-67-71
272 - Adam Scott (AUS) 71-66-69-66, Rory McIlroy (NIR) 67-67-72-66, Jordan Spieth 67-70-68-67
273 - Graham DeLaet (CAN) 68-68-69-68
274 - Angel Cabrera (ARG) 71-72-66-65, Charl Schwartzel (RSA) 72-66-70-66, J.B. Holmes 71-68-67-68, Chesson Hadley 68-70-68-68
275 - Kevin Chappell 68-72-70-65, Ernie Els (RSA) 70-69-69-67, Bill Haas 72-68-67-68, Martin Kaymer (GER) 68-70-64-73
276 - Camilo Villegas (COL) 70-71-68-67, Jimmy Walker 72-67-69-68, Hideki Matsuyama (JPN) 69-67-71-69
277 - Henrik Stenson (SWE) 68-69-72-68, John Senden (AUS) 73-66-70-68, Gary Woodland 67-73-68-69, Noh Seung-Yul (KOR) 70-71-67-69, Ben Crane 70-70-68-69, Russell Knox (SCO) 74-69-72-62
278 - Jerry Kelly 71-73-66-68, Daniel Summerhays 75-68-71-64