The 33-year-old shot a four-under-par 68 that included five birdies to move to 13 under overall.
Jason Dufner had led by five stokes before Saturday's third round, but carded five over for the day to drop to tied third - four shots off the lead.
Americans Bubba Watson and Justin Thomas are also on nine under par.
Dufner ran up four early bogeys at Muirfield Village and compounded his misery by driving into a water hazard at the last for a closing bogey.
Former world number ones Rickie Fowler, at eight under, and Jordan Spieth, a stroke further back, remain in contention.
Daniel Summerhays holds a three-shot lead over fellow American Matt Kuchar heading into the final round of the Memorial Tournament in Ohio.