In pursuit of his first major victory after several close calls over the past five years, Day carded 66 in receptive conditions at Whistling Straits to post a 15-under total of 201.
The world number five picked up six shots in six holes from the ninth, and then rebounded from a double-bogey at the 15th by draining a curling 24-footer at the 17th before parring the last.
That left Day two ahead of Masters and U.S. Open champion Jordan Spieth, who reignited his hopes of a rare third major victory this year by firing a bogey-free 65 on a hot and steamy afternoon.
English world number six Justin Rose (68) was a further stroke back at 12 under, level with South African Branden Grace, who spectacularly holed out from a greenside bunker to birdie the last and card a best-of-the-day 64.
Germany's Martin Kaymer, winner of the 2010 PGA Championship played at Whistling Straits, drained three birdie putts of at least 30 feet as he shot a 65 to finish at 11-under.
Unheralded Australian Matt Jones, two ahead after the storm-delayed second round finished earlier in the day, briefly moved three strokes clear before he unravelled over the closing stretch on the way to a 73 and a tie for sixth.
(Editing by Andrew Both)
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