The defending champion shot a six-under-par 66 for an 11-under halfway total, level with fellow Americans Bill Haas and Jimmy Walker.
England's Paul Casey, who shared the lead after round one, lost ground with a 70 and is in a tie for seventh.
Casey is on eight under alongside Matt Kuchar and Open champion Zach Johnson.
In between Casey and the leaders are three more Americans - Chris Kirk, Patrick Reed and Bubba Watson - all on 10 under, one shot off the three pacesetters.
Casey's compatriot Justin Rose is significantly off the pace after a 72 left him 10 strokes behind the leaders.
World number one Jordan Spieth moved into a three-way tie for the lead after two rounds of the World Challenge at Albany in the Bahamas.