Golf: Phil Mickelson holds one-shot lead heading into PGA Championship final round, on track to become oldest Major winner

Phil Mickelson looked ready to run away with the PGA Championship until a major stumble saw his five-shot cushion evaporate but he steadied the ship in time to sit one shot clear of Brooks Koepka after the third round on Sunday (NZ time).

The 50-year-old Mickelson, who is bidding to become the oldest major winner in history, carded a two-under-par 70 that left him at seven under on the week and one shot clear of Koepka (70), who had a share of the lead until a closing bogey.

Mickelson began the day at the Ocean Course with a one-shot share of the lead with South African Louis Oosthuizen (72) but made a sizzling start with four birdies over the first seven holes and led by five shots after a birdie at the 10th.

However, things started to unravel at the par-four 12th hole where Mickelson, who won the most recent of his five majors at the 2013 British Open, made his first bogey in 21 holes.

Mickelson's tee shot at the par-four 13th then found water en route to a double-bogey that left him just one shot in front of Koepka and Oosthuizen.

Four-times major champion Koepka, playing in the penultimate pairing, moved into a share of the lead with a tap-in birdie at the par-five 16th but fell one back after two-putting from about six feet at the last.

"I'm right where I want to be, and we'll see how tomorrow goes," said Koepka, who had surgery in March to fix a dislocated kneecap and ligament damage. 

"Just be within three of the lead going into the back nine and you've got a chance."

Masters champion Matsuyama (76), in his second tournament since becoming the first Japanese man to win a major, had been in contention until a disastrous stretch that saw him cover the final eight holes in six over par.