Dustin Johnson has taken the outright lead at the BMW Championship after shooting a third-round 68 in Indiana.
The world number two failed to match his form from Friday, when he hit five birdies and two eagles to card a course record, but he did enough to hold off Roberto Castro.
Castro, a joint leader at the end of round two, dropped two shots and failed to register a single birdie on a tough afternoon as he slipped down the leaderboard into fourth.
England's Paul Casey was among those to capitalise, with four birdies leaving him three shots behind leader Johnson heading into the final round as he seeks a first win of the season.
JB Holmes occupies third place following a third-round 68, while elsewhere Rory McIlroy and Graeme McDowell shot rounds of one-over to drop 16 places to joint 35th.
