NORTON, September 1, 2014 (AFP) - The 29-year-old Kirk posted the biggest win of his career and moved into top spot in the FedEx Cup playoff standings.
"I really felt good with the putter today. My biggest win ever," said Kirk, who reached 15-under 269 at TPC Boston. "I am very happy with my win today.
"I may look calmer than some other guys in that situation, but I am still freaking out on the inside."
Kirk, who jumped to the top of the FedEx Cup playoff points list with two events to play, posted his first victory since last year's McGladrey Classic.
Horschel needed a birdie to tie or an eagle to win heading to the par-five last hole. He put his drive in the middle of the fairway, giving him a shot to make the green in two.
But he chunked his second shot, which landed well short and in a hazard. Horschel (69) settled for a bogey and fell into a three-way tie for second.
He was joined at 13-under 271 by Geoff Ogilvy (65) and third-round leader Russell Henley (70).
"I was just trying to make sure I hit a good golf shot and I hit it a little too long," Horschel said. "There are a lot of positives to take away from this week.
"It is the first time I've been in contention all year. I handled myself really well, stayed calm and stayed patient. I made one bad swing and it happened to come on the last hole."
This year's Open champion and PGA Championship winner Rory McIlroy, who won this event last year, shot a one-under 70 to finish tied for fifth with John Senden (66) at 273. McIlroy, who entered the FedEx Cup playoffs in top spot, remained number two in the standings.
Jason Day, who entered the day two shots back along with Kirk and McIlroy, carded a final-round 71 and was tied for seventh with this year's US Open champion Martin Kaymer, of Germany, at 274.
Entering the week with the top spot in the FedEx Cup standings after his victory at The Barclays, Hunter Mahan finished in sole possession of 64th at one-over 285 following a 68 on Monday. He is now third in the standings behind Kirk and McIlroy.
Leading scores on Monday from the fourth round of the US PGA Tour's Deutsche Bank Championship (par-71, USA unless noted):
269 - Chris Kirk 73-66-64-66
271 - Geoff Ogilvy (AUS) 70-71-65-65, Billy Horschel 69-66-67-69, Russell Henley 70-66-65-70
273 - John Senden (AUS) 69-71-67-66, Rory McIlroy (NIR) 70-69-64-70
274 - Martin Kaymer (GER) 71-66-70-67, Jason Day (AUS) 66-68-69-71
275 - Carl Pettersson (SWE) 67-73-69-66, Jimmy Walker 70-70-68-67, Webb Simpson 66-70-68-71, Bill Haas 67-69-70-69, Noh Seung-yul (KOR) 69-68-68-70, Robert Streb 73-67-67-68, Chesson Hadley 66-73-67-69
276 - Adam Scott (AUS) 73-68-68-67, Kevin Stadler 71-70-67-68, Ryan Palmer 63-71-71-71, Zach Johnson 71-68-70-67, Jason Kokrak 68-72-70-66, Brian Stuard 72-71-65-68, Keegan Bradley 65-71-69-71