Jordan Spieth stormed into contention for an historic third major title in 2015 after an extraordinary third round of the US PGA Championship at Whistling Straits.
Spieth was five shots behind with just two holes to play as Jason Day threatened to take control of the championship with six threes in succession from the ninth, including four birdies and an eagle.
However, the gap was suddenly down to two when Spieth birdied the 17th as Day was taking two shots to get out of bunker to double bogey the 15th, and it was down to one when Spieth carded his sixth birdie in eight holes to complete a stunning back nine of 30 and a flawless 65.
Day responded superbly with a birdie from 25 feet on the 17th to shoot 66 and finish 15 under par, two shots clear of Spieth with England's Justin Rose and South African Branden Grace a shot further back.
Rose recovered from a double bogey on the fourth for the second day running with six birdies before a bogey on the 18th in his 68, while Grace holed a bunker shot on the last for his eighth birdie in a flawless 64.
Former champion Martin Kaymer, who won here in 2010, is four off the lead after a bogey-free 65 playing alongside Grace, while halfway leader Matt Jones played his second shot on the ninth from a hospitality tent on his way to a 73 to finish 10 under.
Masters and US Open champion Spieth is now an incredible 50 under par for this year's majors as he looks to become only the second player after Ben Hogan and Tiger Woods to win three majors in a season - Woods was 53 under in winning the US Open, Open and US PGA in 2000.
And the 22-year-old can also replace Rory McIlroy as world number one with a victory if McIlroy, nine off the lead after a 68, does not finish outright second.
"I don't think it gets any easier but it does feel a bit different each time you go through it, you get a bit more comfortable," said Spieth, who finished a shot outside the play-off in the Open Championship as he chased the third leg of an unprecedented calendar grand slam at St Andrews.
Day has recorded nine top-10 finishes in his last 19 majors, memorably battling attacks of vertigo to finish ninth in the US Open in June and, like Spieth, missing a birdie putt on the 72nd hole at St Andrews to get into a play-off.
"Everyone is trying to win. I have to do the best I can and stay out of my own way," Day said. "There are guys that are great players trying to catch me but I have to just let things happen and be patient with myself."
