Mickelson, who held a two-shot advantage after 36 holes, carded a double-bogey on the 15th and a bogey on the next hole, but finished with a pair of pars to end with a 1-over-par 73.
Watney, meanwhile, stumbled to a double-bogey of his own on the 17th at Quail Hollow, finishing with a 1-under 71 to remain tied with Mickelson at 8-under 208.
Grabbing sole possession of third place at 7-under was George McNeill (72).
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