Wyndham Clark carded a flawless second round to reach 12 under and lead the BMW Championship in St Louis by one stroke over Gary Woodland. The American is the only player without a bogey through 36 holes.

Overnight co-leader Rory McIlroy slipped to five shots behind after a one-under 69. The Northern Irishman opened with a six-under 65 but struggled to replicate that form on Friday. "I didn't quite hit it as good as I did yesterday," McIlroy told the Golf Channel, adding he would spend the afternoon on the driving range.

Despite the drop, McIlroy remained positive about his position. "I'm proud of how I just hung in there. A lot of good par putts on the back nine, especially," he said. His only bogey came at the par-five eighth, but strong par saves kept him in contention at seven under alongside Collin Morikawa and Russell Henley.

Justin Rose sits four under, while Robert MacIntyre and Matt Fitzpatrick are three under.