The US Open champion shot two eagles and five birdies to move to 14 under with fellow American Robert Castro, who posted a second successive 65.
England's Paul Casey, who went round in 66, is three shots adrift in third.
Northern Ireland's Rory McIlroy and American Jordan Spieth are tied for 19th on four under after rounds of 72.
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Dustin Johnson carded a course record nine-under-par 63 to share the lead after two rounds of the BMW Championship in Carmel, Indiana.