New York - British Open champion Jordan Spieth turned up the heat on the back nine again on Saturday to seize a three-stroke lead over world number one Dustin Johnson at the Northern Trust.
Spieth fired four of his eight birdies coming in -- including three in a row from the 14th through 16th -- to card a six-under 64 for a 12-under par total of 198.
His lead would have been a stroke greater but for Johnson's birdie on the 18th, where he stuck his second shot four feet from the pin to cap a 67 for 201.
England's Paul Casey (66), Spain's Jon Rahm (67) and Americans Patrick Reed (66) and Matt Kuchar (68) were a further two strokes back.
Spieth's six-under round matched the low round of the tournament at Glen Oaks Club in Old Westbury, New York.
"I didn't drive the ball quite as well as I did the first two days, and I'm going to need to tomorrow to make it a little easier on myself," the 24-year-old Texan said.
"I got away with missing some fairways on that front nine with some nice putts."
But even with Johnson chasing him, Spieth is in a commanding position. He has never lost on the PGA Tour when leading by two or more strokes going into the final round.
"It's easier to win from that position," Spieth said, although he noted that it wasn't always smooth sailing.
Spieth took a three-stroke lead into the final round of the Open Championship at Royal Birkdale this year and saw it evaporate before he turned things around for the win.
"Anything can happen on Sunday," he said.
Johnson was confident of the same thing.
"I'm just hitting it really well," Johnson said. "I feel like I can control my distance very good. I'm getting a lot of really good looks at birdie ... I just need to keep doing what I'm doing."
The tournament is the first of four events in the US PGA Tour's FedEx Cup playoffs. The series ends with the Tour Championship, where a $10 million bonus is on offer to the series points winner.
Leading third-round scores on Saturday in the Northern Trust, first event of the US PGA Tour FedEx Cup playoffs (USA unless noted):
198 - Jordan Spieth 69-65-64
201 - Dustin Johnson 65-69-67
203 - Paul Casey (ENG) 69-68-66, Patrick Reed 70-67-66, Jon Rahm (ESP) 68-68-67, Matt Kuchar 71-64-68
204 - Keegan Bradley 68-70-66
205 - Kevin Chappell 68-73-64, Justin Rose (ENG) 68-68-69
206 - Jason Dufner 69-69-68, Justin Thomas 68-69-69, Chez Reavie 69-68-69, Patrick Cantlay 67-70-69, Russell Henley 64-72-70, Bubba Watson 67-68-71, Jhonattan Vegas (VEN) 69-65-72
207 - Robert Streb 71-70-66, Scott Brown 66-74-67, David Lingmerth (SWE) 71-68-68, Louis Oosthuizen (RSA) 67-72-68, Martin Laird (SCO) 69-69-69
208 - Henrik Stenson (SWE) 69-72-67, Jason Day (AUS) 69-71-68, C.T. Pan (TPE) 70-69-69, Kim Whee (KOR) 69-69-70, Harold Varner 67-70-71, Rickie Fowler 68-66-74