ATLANTA, 26 Sept 2015, (AFP) - In torrential rain at the East Lake Golf Course, Spieth closed out a solid round in difficult conditions with a superb curling 20-foot birdie putt on the 18th to cap a battling third round performance.
Spieth will pocket a $1.48 million winner's check as well as a $10 million bonus for clinching the FedEx Cup playoff series if he hangs on to his lead in Sunday's finale to the USPGA season.
"It was a fantastic day," Spieth said. "I would have taken even par when the day started and so to finish a couple under was fantastic."
But it was a frustrating third round for overnight leader Stenson, who saw his three-shot lead evaporate after a scratchy two-over-par 72 which included four bogeys and only two birdies.
Stenson looked to have overcome a wobble on the back nine, when he posted consecutive birdies on the 10th and 11th before a birdie on the 14th restored his two-shot cushion.
However Spieth notched a birdie on the 15th to halve Stenson's lead and when the Swede bogeyed the 17th the two rivals headed to the 18th as joint leaders before Spieth produced another birdie to take sole ownership on eight-under 202.
Stenson admitted he had struggled in the grimy conditions which he likened to a muddy endurance race.
"Wet, muddy, rough," Stenson said. "What is that race called, Tough Mudder? Is that what it's called? That's what it felt like.
"All in all, it was a tough day and I didn't play my best, but I still think I kept it together quite nicely in tough conditions."
Stenson, on 203, has a three-shot advantage over his nearest rivals, with Rickie Fowler and England's Paul Casey tied for third on four under.
Fowler produced a scintillating back nine performance which included four birdies to finish with a three-under-par 67.
"With the conditions, I hit some great shots, made some good putts and it's going to go a long ways to put myself in good position for Sunday," Fowler said.
Northern Ireland's Rory McIlroy was a further shot back on three under alongside Zach Johnson after carding an even-par 70.
McIlroy made a shaky start with bogeys on the first and fourth holes before he suddenly starting finding his putting range.
He rolled in a 26-foot effort on the seventh for his first birdie and then followed it up with a 21-footer on the ninth to go to the turn at level par.
A fine approach on the 12th left him a two foot tap-in for a third birdie before an 18-foot birdie putt on the 13th took him to two under.
A bogey on the 14th, however, checked his momentum before another birdie on 15 put him within sight of the lead.
A bogey on the 16th followed by a birdie on the 17th summed up his day, and a disastrous double-bogey on the par-three 18th dropped him back to level par for the day.
Elsewhere Saturday world number one Jason Day struggled to mount a challenge to the front-runners, with two bogeys and two birdies leaving him with a par 70, level par after three rounds, eight shots off the lead.
Third-round scores on Saturday in the $8.25 million US PGA Tour Championship, final event in the FedEx Cup playoff series (USA unless noted, par 70):
202 - Jordan Spieth 68-66-68
203 - Henrik Stenson (SWE) 63-68-72
206 - Rickie Fowler 69-70-67, Paul Casey (ENG) 65-70-71
207 - Rory McIlroy (NIR) 66-71-70, Zach Johnson 66-70-71
208 - J.B. Holmes 68-72-68
209 - Bubba Watson 70-71-68, Justin Rose (ENG) 70-68-71
210 - Daniel Berger 69-73-68, Danny Lee (NZL) 69-72-69, Hideki Matsuyama (JPN) 69-72-69, Matt Kuchar 71-70-69, Jason Day (AUS) 69-71-70, Steven Bowditch (AUS) 68-69-73
211 - Kevin Na 68-73-70
212 - Dustin Johnson 69-72-71
214 - Brooks Koepka 68-74-72
215 - Charley Hoffman 73-72-70, Bae Sang-Moon (KOR) 73-70-72, Brandt Snedeker 68-72-75
217 - Bill Haas 72-73-72
218 - Harris English 71-76-71, Jimmy Walker 73-71-74
219 - Robert Streb 75-75-69
220 - Scott Piercy 74-73-73
223 - Patrick Reed 72-74-77
225 - Kevin Kisner 76-77-72
WD - Louis Oosthuizen (RSA)
DNS - Jim Furyk