Dustin Johnson set the course record at Crooked Stick to take a share of the lead at the BMW Championship after a second-round nine-under 63.
The US Open champion had two eagles and five birdies as he joined Roberto Castro at the top of the leaderboard on 14 under after Castro's second successive 65.
Paul Casey shot a six-under 66 to move into third place on 11 under.
Johnson was frustrated of late with his putting so switched up putters for this tournament and told reporters: "I was just struggling to get the ball to come out on the line I was seeing.
"No matter what I did, how much I practiced, training aids, whatever. It just wasn't working. So just decided to switch it up."
Castro sunk three straight birdies on the front nine to put himself into a big lead early on but Johnson holed from the bunker for eagle on the ninth before a birdie on the 11th.
He then surged up the leaderboard with a 10-foot birdie putt on the par-three 13th, a 30-foot birdie putt on the 14th and an eagle from 25 feet on the 15th.
Rory McIlroy struggled with three bogeys on the back nine for a 71 and sits in a tie for 19th place on four under, while Jordan Spieth had four bogeys and a double bogey as he signed for a 72 to join McIlroy.