CHERRY HILLS VILLAGE, September 6, 2014 (AFP) - While McIlroy's assault threatened to dissolve at the par-three 12th, Horschel vaulted three strokes clear of the field, ending 54 holes at 13-under-par 197 at Cherry Hills Country Club in Colorado.
US Open champion Martin Kaymer (64) and Masters winner Bubba Watson (66) share third place at eight-under 202, five shots adrift of Horschel but still in the hunt.
Ryan Palmer, who was runner-up at the Humana Challenge and The Honda Classic, posted a 67 and is alone in second on 200. Rickie Fowler shot 66 and is in fifth.
But the day belonged to the 27-year-old American Horschel, who is on real form, having shared second place last week at the Deutsche Bank Championship.
Horschel, of Jacksonville Beach, Florida, matched this tournament's 54-hole scoring record with his total of 197. Camilo Villegas set the mark in 2008 at Bellerive golf course and Tiger Woods matched it a year later at the Cog Hill course.
Second-round leader Sergio Garcia, of Spain, stumbled to a two-over 72 and fell into sixth place at 204.
McIlroy didn't fare much better. The Northern Irishman ended up posting a 72, which dropped him into a tie for 10th at four-under 206.
Phil Mickelson, who shared second place in 2012, withdrew before the third-round started, ending his PGA Tour season, and Keegan Bradley also pulled out, citing a rules violation during the first round.
Leading scores on Saturday after the third round of the PGA Tour's BMW Championship (par 70, USA unless noted):
197 - Billy Horschel 68-66-63
200 - Ryan Palmer 69-64-67
202 - Bubba Watson 70-66-66, Martin Kaymer (GER) 68-70-64
203 - Rickie Fowler 71-66-66
204 - Sergio Garcia (ESP) 68-64-72
205 - Jim Furyk 70-68-67, Graham DeLaet (CAN) 68-68-69, Jordan Spieth 67-70-68
206 - Adam Scott (AUS) 71-66-69, J.B. Holmes 71-68-67, Rory McIlroy (NIR) 67-67-72, Morgan Hoffmann 72-72-62, Chesson Hadley 68-70-68
207 - Bill Haas 72-68-67, Hideki Matsuyama (JPN) 69-67-71
208 - Ben Crane 70-70-68, Ernie Els (RSA) 70-69-69, Charl Schwartzel (RSA) 72-66-70, Jimmy Walker 72-67-69, Gary Woodland 67-73-68, Noh Seung-Yul (KOR) 70-71-67