Rory McIlroy falls back as brilliant Max Homa takes BMW Championship lead
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McIlroy had shared the lead with Brian Harman after a flawless first-round 65 at Olympia Fields but his level-par 70 on Friday left him five shots back.
Homa had 10 birdies and two bogeys in a stunning course-record 62 and holds a two-shot lead over fellow American Chris Kirk heading into the weekend.
McIlroy birdied the third and parred the next 11 holes before coming unstuck on the par-five 15th when he drove into the trees on his way to a bogey.
The 34-year-old told Sky Sports: “(My round was) pretty mediocre. One birdie, one bogey. Gave myself tons of chances.
“I felt like I hit good putts, just hit a lot of edges and the ball just sort of slid by.
“I don’t feel like I played too different to how I played yesterday, I just got a bit more out of my round yesterday.”
England’s Matt Fitzpatrick is a further shot off the pace in a tie for third with Harman after a bogey on the 18th saw him post a three-under 67.
Justin Rose had eight birdies and three bogeys in his second-round 65 to join McIlroy and American trio Scottie Scheffler, Rickie Fowler and Harris English on five-under par for the tournament.
Defending champion Patrick Cantlay is a further shot back on after a second successive 68 while fellow American Xander Schauffele joined him on four under after birdying five of his last 10 holes to sign for a 65.
World number five Viktor Hovland is in a large group on three under but world number three Jon Rahm struggled with a four-over 74 which left him two over for the FedEx Cup play-off event.