Mickelson won the Phoenix Open in February for the 41st tour triumph of his career, but has not been a factor in his three previous events.
He missed the cut at the Arnold Palmer Invitational, wound up tied for 16th at the Houston Open (where he was defending champion) and was a disappointing 54th at the Masters.
Mickelson recorded six birdies at the Quail Hollow Club Friday, shot a 5-under-par 67 that tied for the low round of the day and turned in a 36-hole score of 9-under 135.
Rory McIlroy, who was part of a seven-way tie for the lead after the first round, rallied from a slow start Friday and shot a 71 that left him in a tie for fifth place at 138 with Lee Westwood, Rod Pampling, Jason Kokrak and Derek Ernst.
Tiger Woods is the only player with more than one victory on the tour this year, having won three times