Mickelson fired a 9-under 62 Saturday to finish three rounds at 16-under 197, one stroke off the 54-hole tournament scoring record.
Andres Romero birdied the final three holes to move into second place at 12-under 201 after a third-round 65.
Second-round leader Scott McCarron had a 1-under 70 Saturday that dropped him into a tie for third place at 11-under 202. He was joined in third by Rory Sabbatini (67), K.J. Choi (67) and U.S. Presidents Cup captain Fred Couples (65).
"The more opportunities that I get in contention, the better off I am," Mickelson said following Saturday's round. "I haven't been in contention the first three weeks and it has been frustrating. Now that I have a good chance going into the final round, it's fun."