The Masters champion followed his opening-round 64 with a more difficult day in St Louis, mixing four-birdies with a bogey as he slipped several shots behind the leaders.
Rory McIlroy has fallen off the lead at the BMW Championship after a second-round 69 left him seven-under-par heading into the weekend.
The Masters champion followed his opening-round 64 with a more difficult day in St Louis, mixing four-birdies with a bogey as he slipped several shots behind the leaders.
McIlroy admitted he was not as sharp as he had been on Thursday and said he planned to head straight to the driving range after his round to work on his swing.
American Wyndham Clark leads the tournament on 12-under after consecutive rounds of 64, with Gary Woodland one shot further back.
JJ Spaun also remains in contention after another strong round, while world number one Scottie Scheffler recovered from a slow start to move to three-under overall.
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