KOHLER, 15 Aug 2015, (AFP) - Australia's Day was sailing home at Whistling Straits after draining a 13-foot eagle putt at the par-five 11th followed by birdies at 13 and 14 that moved the Australian to 16-under.
But he needed two shots to get out of a bunker en route to a double-bogey at 15, that dropped him back to 14-under just as Spieth was putting the finishing touches to a seven-under 65 for 13-under 203.
Day responded with a 24-foot birdie at the par-three 17th before parring the last.
Spieth, the Masters and US Open champion, birdied six of his last eight holes for sole possession of second place, boosting his bid to join Ben Hogan and Tiger Woods as the only men to win three major championships in one year.
It was a further stroke back South Africa's Branden Grace, who holed out from a bunker at the 18th to cap an eight-under par 64 for 204.
He was joined on 12-under by England's Justin Rose, who bogeyed the last to fall into the tie for third at 12-under with a four-under 68.
Germany's Martin Kaymer, trying to reprise his 2010 PGA Championship victory at Whistling Straits, also made the most of ideal scoring conditions, firing seven birdies in a seven-under 65 for 11-under 205.
Second-round leader Matt Jones was 14-under through 14 but dropped four shots in the next three holes with a two bogeys and a double-bogey. He finished with a one-over 73 for 206, where he was joined by American Tony Finau (69).