Chicago - Jason Day continued his march to try and claim the world number one ranking, stretching his lead to six shots after the third round of the BMW Championship on Saturday.
The 27-year-old Australian remained on course for a wire-to-wire victory as he fired a two-under 69 to reach 20-under 193 in the third of four tournaments in the PGA Tour's FedEx Cup playoffs.
"I am still trying to push forward and stay aggressive," Day said. "I had some mental errors out there today.
"So with how I played and to shoot 69, and expand lead to one more shot, was nice."
Day has a six-shot cushion over Americans Scott Piercy (67) and Daniel Berger (70) who are tied for second at the Conway Farms Golf Club.
World number one Rory McIlroy birdied three of the last four holes for a four-under 67 and is in sole possession of fourth, seven shots back of Day.
Day wobbled down the stretch, making two of his four bogeys on the back nine after making just two in the first two rounds.
But Day, who opened with rounds of 61 and 63, grinded it out and capped his third round by draining a long birdie putt on the last.
Rickie Fowler said Day is in a league of his own.
"Look at Jason, he is in control," said Fowler. "I don't worry about him. There is another tournament going on. Playing the way he is there is not a lot we can do unless the tournament goes seven, eight, nine, 10 rounds."
Day, of Forest Lake, Queensland, could get to world number one if he claims his fifth title of the season on Sunday. He is trying to join Tiger Woods and Vijay Singh as the only players in the last 20 years to win five or more tournaments in a season.
McIlroy will stay number one if he wins Sunday.
Jordan Spieth, who shot a 72 and is tied for 11th, can also claim number one with a win but was 10 shots off the pace.
Day began his round with back to back pars before making birdie on the par-four third. He played the final four holes of the front nine by offsetting two bogeys with a couple of birdies.
He had two bogeys in a five-hole stretch beginning on 13, but he managed to right the ship by draining a 25-foot putt on the par-five 18th for his sixth birdie of the day.
Day is aiming for his fourth title in his past six tournaments, a stretch that includes his first career major win at the PGA Championship and the first event of the FedEx Cup Playoffs, The Barclays.
"I am going to try and trust myself a little bit better tomorrow. I hope it happens," Day said.
He also has an opportunity to pass reigning US Open and Masters champ Spieth for player of the year honours.
Fowler had six birdies in a 66 and was in a tie for fifth with Dustin Johnson, who had a 68, and Kevin Na, who birdied the last two holes for a 70.
Leading third-round scores on Saturday in the $8.25 million BMW Championship (USA unless noted, par-71):
193 - Jason Day (AUS) 61-63-69
199 - Scott Piercy 67-65-67, Daniel Berger 65-64-70
200 - Rory McIlroy (NIR) 68-65-67
201 - Rickie Fowler 69-66-66, Dustin Johnson 71-62-68, Kevin Na 65-66-70
202 - J.B. Holmes 70-65-67, Harris English 65-68-69, Justin Thomas 65-67-70
203 - Kevin Chappell 66-69-68, Brendan Steele 68-67-68, Brendon de Jonge (ZIM) 67-67-69, Jordan Spieth 65-66-72
204 - Justin Rose (ENG) 70-64-70, Matt Kuchar 67-67-70, George McNeill 67-65-72
- Louis Oosthuizen (RSA) 71-66-68, Hunter Mahan 68-68-69, Cameron
Tringale 72-64-69, Hideki Matsuyama (JPN) 72-63-70, Nick Watney
68-66-71, Henrik Stenson (SWE) 71-63-71, Brendon Todd 66-63-76
206 - Patrick Reed 68-69-69, Keegan Bradley 68-66-72
- Paul Casey (ENG) 67-69-71, Sergio Garcia (ESP) 70-65-72, Brian Harman
66-69-72, Bubba Watson 65-70-72, Ryan Palmer 67-67-73