Dustin Johnson of the US plays a second shot on the 18th hole during the third round of the BMW Championship on Saturday in Olympia Fields, Illinois. Photo: VCG

World No.1 Dustin Johnson and Japan's Hideki Matsuyama will share a two-stroke lead going into the final round of the BMW Championship on Sunday after shooting one-under-par 69s on Saturday.

The Olympia Fields Country Club, where 69 players are battling it out for 30 spots in this week's Tour Championship, continued to torment the world's best golfers and the leading pair were alone in having tournament totals under par.

Their tally of one-under 209 gave them a bit of breathing space at the top of a crowded leaderboard ahead of Australia's Adam Scott (70), Canadian Mackenzie Hughes (69) and Chilean Joaquin Niemann (68), who shared third place.

Johnson tore up the TPC Boston for an 11-shot victory in the previous week's Northern Trust Open - the first of three FedExCup playoff events - and the American thought he had hit upon the right strategy for success at the trickier Illinois course.

"Felt like I hit a lot of fairways, a lot of greens, and it really doesn't matter how far away you are, you've just got to be in the fairway," he told reporters.

"That's the only way to control the ball and control the spin, and even then it's still difficult to get it close to the hole."

Johnson, who will extend his lead at the top of the FedExCup standings if he can snare a third BMW Championship title on Sunday, mixed three birdies with two bogeys.

Matsuyama made the perfect start with an eagle at the par-five first hole but two birdies against three bogeys over the rest of the round deprived him of the outright lead.

Rory McIlroy, who shared the overnight lead, fell off the pace and into a share of sixth with five others on two-over after shooting a 73, later explaining that the imminent arrival of his first child had been a distraction.

Spain's Jon Rahm also shared sixth after carding the joint lowest round of the day with a five-birdie 66 that would have been better had he not ­forgotten to put a marker on the fifth green before picking up his ball. He was docked a stroke.

"I was holding my marker in my pocket, just went at it, and for some reason I just picked up the ball thinking I marked it already," said Rahm.

Tiger Woods sits in a share of 55th heading into the final round after a 72 on Saturday.