By Andrew Both

FARMINGDALE, N.Y. (Reuters) - American Brooks Koepka moved inexorably closer to his fourth major title, maintaining a seven-stroke lead halfway through the third round at the PGA Championship on Saturday in what was both literally and figuratively a walk in the park.

After knocking his rivals to the canvas on Friday, when he opened up a seven-shot halfway lead, Koepka remained in charge with a near-clinical front nine at Bethpage Black.

The defending champion played with machine-like precision, smashing his drives more than 300 yards and hitting soaring iron shots that honed in on the pins as if attracted by magnets.

He gave himself good birdie chances on each of the first six holes and though he converted only two of the putts, he did what he needed to do in what had the makings of a 36-hole weekend coronation of a new world number one.

Even though he three-putted the ninth for his first bogey of the day, Koepka remained in control.

His rivals simply had too much ground to make up on the leader, who was 13 under par as he made the turn at the far side of the course.

Perennial major contender Dustin Johnson and Thailand's Jazz Janewattananond were equal second on six under after 10 and 12 holes respectively, with a further two-shot gap to the best of the rest.

MCILROY AN AFTER-THOUGHT

Earlier, as Koepka prepared to tee off, Rory McIlroy was already done with his day, reduced to an after-thought at a tournament he entered as the best player in the world this year, a mantle he will surely hand over to Koepka on Sunday.

McIlroy shot a one-under-par 69 that would have been significantly better with a co-operative putter but, for someone whose career revolves around the majors, it was a jarring experience to tee off among the dew-sweepers.

But the four-times major champion, who made the halfway cut with nothing to spare, said he had not earned the right to a later tee time, as the scorecard spoke for itself.

"I haven't played well enough to be out there, and that's the way it is," the Northern Irishman said when asked if he wished he was in Koepka's position.

"I feel that would be very entitled (to think that).

"It's awesome (the way Koepka is playing). I watched most of it yesterday afternoon. He's definitely, in these events, playing on a different level than most anyone else."

Saturday dawned magnificent on Long Island, with bright sunshine and a light breeze helping to dry out a course drenched with rain earlier in the week.

McIlroy was the big early attraction for the large Long Island galleries and he gave them something to cheer with an eagle at the par-five fourth, where he sank a 30-foot putt.

After missing a seven-foot birdie putt at the 15th he was clearly frustrated on the 16th tee, distracted by spectators, nearly all of whom seemed more interested in capturing him with their smartphones than actually watching.

"Guys, can you turn your phones down," McIlroy said in quiet exasperation.

Less than an hour later he was done, signing for a two-over 212 total.

(Reporting by Andrew Both; editing by Clare Fallon)