Jason Day held a three shot lead as the final round of the US PGA Championship approached its climax at Whistling Straits.
The 27 year old brought a two shot lead over history chasing Jordan Spieth into the final round and reached the turn in 33 to get to 18 under and move to within touching distance of a first Major Championship.
Branden Grace had provided the closest competition over the front nine but recorded a double bogey on the tenth to drop to 14 under while playing partner Justin Rose birdied the same hole to become the nearest challenger.
Masters Tournament and US Open champion Spieth, looking to become just the third man to win three Majors in the same year, was alongside Grace after making the turn in 35 with Anirban Lahiri a further shot back after 11 holes.
Day and Spieth parred the first but when Day got up and down from a greenside bunker on the second he had opened up a three shot lead.
Spieth responded with a birdie of his own on the third but the chasing pack refused to be left behind as Grace birdied the same hole.
A bogey from Spieth on the fourth briefly reopened Day's three shot lead but Grace nailed a 35 footer on the fifth to leapfrog the American into second at 14 under.
Grace followed that with a birdie on the sixth and Rose, who had parred his first four holes, also picked up shots on the fifth and sixth.
Day found the bunker off the fifth tee but still managed to make birdie, as did Spieth, and the top four were threatening to pull away from the rest of the field despite Lahiri reaching the turn in 32.
The scorching pace of scoring continued as Grace made a 17 foot putt for a third consecutive gain but back on the sixth both Day and Spieth picked up further shots, the latter getting up and down from some thick rough.
Everything Day touched seemed to be finding the hole and he drained a spectacular 50 footer on the seventh to make it three birdies in a row himself and move three clear of Grace at 19 under.
The Australian found a fairway bunker on the eighth and gave a shot back but saved par on the ninth after a poor second while Spieth dropped a shot after finding the rough.
Ahead of them, Grace also found rough off the tee and after failing to get his third shot on the green, gave two back as Rose put his second inside six feet for birdie.
Brooks Koepka was 11 under through 13 holes, George Coetzee was ten under having played one more while Martin Kaymer had fallen off the pace and was one over for the day after ten to sit ten under.
World Number One and defending champion Rory McIlroy began with five pars but produced a brilliant up and down after finding trouble off the tee on the par four sixth to make his first birdie of the day.
Further gains followed on the seventh and tenth but the two time winner then dropped a shot on the 13th and was eight under with three to play.
Swede David Lingmerth ended a good week with a 66 to set the clubhouse target at ten under while Victor Dubuisson registered a second consecutive 67 to finish his week two shots further back.
The Frenchman, who finished in a tie for seventh at Valhalla last year, made three consecutive birdies from the fifth and picked up further shots on the 11th, 12th and 16th before finishing with a bogey on the last.
