The world number one completed a -4 round of 67 on Saturday with six birdies and two bogeys to put him on -13 and ahead of Bernd Wiesberger of Austria.America's Rickie Fowler is another stroke back in third while his compatriot Phil Mickelson, and Australia's Jason Day, are within three of the lead.McIlroy, who has won his last two competitions including his first Open Championship, said he is prepared to scrap it out for a chance to land a second US PGA victory.The 25-year-old said: "I'll take a win any way it comes. If that means having to scrap it out with a couple of people coming down the stretch, so be it. Any win would be very satisfying."McIlroy took a one-shot lead into the third round but didn't have it all his own way at Valhalla as he struggled to reproduce the form which saw him win at Royal Liverpool and lift a first WGC title.The Northern Irishman escaped with a par on the par-four fourth after pulling his drive into a hazard and had to take a drop, before pitching to 11 feet and holing the putt.He was briefly a shot behind Wiesberger when the world number 70 birdied the 16th and almost holed his approach to the 17th, but McIlroy responded in style with birdies on 15, 16 and 17.Rory continued: "I didn't have my best stuff for some of that round today. Obviously I finished well with three birdies in the last four holes but the middle of the round was pretty scrappy."The last two days it has not been quite as good as I can be. If everything does click tomorrow, then obviously I feel like there's a very low score in me."The biggest thing I was concerned about on the back nine today was not being in the lead. I got tied for the lead there for a while and I wanted to just get back in the lead. I'm really confident right now no matter who is on that leaderboard; I feel like I have a pretty good chance in beating them."