World No.1 Scottie Scheffler and U.S Open winner Wyndham Clark, along with Rickie Fowler, Chris Kirk, Sahith Theegala and Matt Fitzpatrick are a shot further back at 66.

Jason Day is the highest-ranked Australian, tied in 18th spot after a one-under opening-round 69, while a horror triple bogey on his final hole left Cam Davis three-over for the day, in joint-45th place, carding 73.

Davis, who on Wednesday spoke about battling through some "really low points" in the early part of the season, was one of only two golfers to move from outside the top-50 in the FedExCup standings to qualify for this week's BMW Championship at the Olympia Fields Country Club.

The 28-year-old Sydneysider arrived in Chicago ranked 47th, with the top-30 players after the completion of this tournament qualifying for the season-ending Tour Championship next week in Atlanta.

McIlroy turned in a bogey-free round despite hitting just three of 14 fairways in regulation on an uncharacteristically poor day with his driver.

At the par-4 17th hole, his drive flew left and his second shot snuck between trees and rolled over the back of the green.

That set him up for a 40-foot pitch shot that gently rolled into the left corner of the cup for his fifth and final birdie of the round.

"It was a bit of a hit and a hope," McIlroy said.

"Just trying to thread that needle and hit it straight at the 17 sign. It's only Thursday, I thought, 'What the heck, I'll take it on and see what happens'."

The defending FedExCup champion, McIlroy entered the week No.3 in the standings with Harman ranked ninth.

Theegala and Fitzpatrick are doing what they can to play their way into the field.

Theegala entered the week No.31 in the standings – so as it stands, he will be the first man to miss out on Atlanta – and Englishman Fitzpatrick is No.40.

Fowler was tied for the lead at five-under when his drive at 17 splashed into a water hazard. He finished the round bogey-par.

"I'm happy with how I drove it for the most part. That's a big key around here," Fowler said.

"If you don't get the ball in play, it's going to be a long day or a long week out there."

Scheffler was even at the turn before birdies on holes 11, 12, 13 and 15 launched him up the leaderboard.