McIlroy was three shots off the lead after the first round -1 R McIlroy (NI), P. Cantlay (US); level D Johnson (US), H Matsuyama (Jpn); +1 T Finau (US), A Scott (Aus), B Horschel (US), L Oosthuizen (SA), B Todd (US) Selected others: +3 T Hatton (Eng); +5 P. Casey (Eng), M Fitzpatrick (Eng); +8 T Woods (USA) Full Leader Board
Rory McIlroy was one of only two players who finished below par at the BMW Championship on Friday when he took some of the overall lead.
The Northern Irishman took a one-under-69 and moved halfway to the top of the standings alongside the American Patrick Cantlay. stage.
McIlroy made five birdies trying to improve on just one top-12 finish since the PGA Tour resumed in June.
"I was able to break par, which is great," he said.
"I got myself in a good place to start the weekend.
" As it got later in the day, the course certainly a little tougher, a little more difficult. The wind rose a little.
"I'm a little disappointed dropping those shots on the way in, but I think if you shoot something below par at this golf course you should be pretty happy."
American Dustin Johnson, who leads the season-long FedEx Cup standings, birded his last two holes at Olympia Fields for a 69 and climbed to a tie for third with Japan's Hideki Matsuyama, who has a three-over 73 shot to reach level-par 140 for the two rounds.
At the end of the game, there were 29 players within five shots of the leaders.
The Briton Tyrrell Hatton, tied for 13th out of three above par, plays among those within easy reach of the top spot.
The BMW Championship is the second of three events in the FedEx Cup playoffs of the US circuit that will determine the PGA Tour champion for this season.
The top 70 players in the season's points standings will participate in this week's event, which will decide the 30 players to participate in the Tour Championship ending next month in East Lake in Atlanta.