Mickelson followed up his opening round of 61 with a 64 on Tuesday to go 17-under for the tournament in Ridgedale, Missouri, with Rod Pampling and Tim Petrovic trailing him going into the final round at Ozarks National.
Mickelson fired five birdies in seven holes on the front nine before a lone bogey immediately after the turn. But he recovered with three more birdies in the next four holes.
He is now in a position to break the PGA Tour Champions' 54-hole scoring record of 191, jointly held by David Frost, Bruce Fleisher, Loren Roberts, Bernhard Langer and Rocco Mediate.
