By Mark Lamport-Stokes

LOUISVILLE Kentucky (Reuters) - Hours before overnight leader Rory McIlroy was scheduled to tee off in the PGA Championship's final round on Sunday, the early starters were going low at a wet, soft and receptive Valhalla Golf Club.

Ireland's Shane Lowry, who set off in the first pairing of the day, birdied six of his first 13 holes before signing off with a four-under-par 67 in the year's final major.

The 27-year-old, a two-time winner on the European Tour, briefly faltered with bogeys on 15 and 16 but ended his round with a flourish, tapping in for a birdie at the last to post a one-under total of 283.

Also taking advantage of the rain-softened greens were twice former PGA Championship winner Vijay Singh, who was three under for the day after 11 holes, and England's Ian Poulter, three under after 10.

However, the main focus for the fans would be the top end of the leaderboard where Northern Irishman McIlroy, hunting his fourth major title and second in a row, held a one-stroke advantage overnight.

The 25-year-old McIlroy, who romped to victory in the 2012 PGA Championship at Kiawah Island by a record eight shots, was set to tee off in the final round at 2:55 p.m. (1455 ET), along with Austrian surprise package Bernd Wiesberger.

McIlroy, an overwhelming favorite after winning in his last two PGA Tour starts, fended off a series of challenges during Saturday's third round by shooting a 67 to stay at the top with a 13-under total.

"It's not the biggest lead I've ever had, but I'm still in control of this golf tournament," world number one McIlroy said after mixing six birdies with two bogeys on Saturday. "It's a great position to be in."

Wiesberger, who has triumphed twice on the European Tour but has never previously fared well in a major, was alone in second at 12 under, with American 'young gun' Rickie Fowler a further stroke back in third.

Phil Mickelson, the 2005 PGA Championship winner, and Australian Jason Day were tied for fourth at 10 under on a tightly bunched leaderboard, setting up the prospect of a thrilling finale.

However, the weather may have a significant influence as heavy rain was sweeping across the course two hours before McIlroy's scheduled tee time and possible thunderstorms have been forecast for later in the day

(Reporting by Mark Lamport-Stokes; Editing by Gene Cherry)