McIlroy’s playing partner Jason Day had to take off his shoes and socks to play his second shot on the second hole – before miraculously escaping with a par fourLouis Oosthuizen, Henrik Stenson, Mikko Ilonen and Ryan Palmer are bunched together as they attempt to cling on to Northern Ireland’s McIlroy.
“It’s not the biggest lead I’ve ever had but I am still in control of this tournament,” McIlroy said. “The two birdies on 15 and 16 were huge.”
The 25-year-old has climbed back to number one in the world rankings after successive wins at the Open Championship and last week’s World Golf Championship event in Ohio.
He took a one-shot lead into the third round at a damp Valhalla, but was unable to reproduce the form which marked him out as the overwhelming favourite to clinch a fourth major win.
A wayward tee shot on the driveable 292-yard fourth, two duffed chips on the eighth and 12th holes, plus an overhit approach shot on the 11th, were uncharacteristic errors by in-form McIlroy.
But he found another gear to finish with three birdies in the final four holes to hold off Wiesberger, describing his own performance as “gutsy”.
He added: “It’s the best place to be in a tournament. I wouldn’t want to be anywhere else.”
Wiesberger, a 28-year-old with only two European Tour wins since turning professional in 2006, had missed the cut in four of his five previous major appearances.
But, playing alongside five-time major winner Mickelson, he finished with three birdies to card an unblemished 65 – the joint lowest score of the day.
“It was a dream come true really, going out there with Phil, one of my heroes,” said Wiesberger, who will tee off alongside McIlroy in Sunday’s final pairing.
“I played beautifully today, I didn’t miss a lot of shots, set up a few nice opportunities on the last few holes especially. I’m very proud of myself the way I played.”
Wales’ Jamie Donaldson shot a five-under 66 to leave him five adrift of McIlroy, boosting his chances of securing a Ryder Cup debut next month.
Donaldson, 38, currently occupies the final automatic spot in Paul McGinley’s team and looks set to earn himself more valuable qualifying points at Valhalla.
“It’s for me to play well and take control and sort it out this week,” said Donaldson. “I’m just trying to go out there and play as well I can and shoot as low a score as I can.”
A shot behind Donaldson is England’s Lee Westwood, who topped the leaderboard after day one as he searches for a first major title at the 67th attempt.
Compatriot Danny Willett made a blistering start, with two birdies and an eagle in the opening four holes, before finishing with a 66 to go six under for the tournament.
Rory McIlroy maintained a one-shot advantage on day three of the US PGA Championship, despite struggling to find his imperious best at Valhalla.
The world number one shot a four-under-par 67 to lead on 13 under par, one clear of Austrian Bernd Wiesberger going into the final day.
Wiesberger finished with three birdies to card a 65, with Rickie Fowler a shot adrift after a 67.
Phil Mickelson and Jason Day are 10 under, with four players on nine under.