A bulletproof Rory McIlroy became an even bigger favourite for the US PGA when he roared back from a mid-round mini-crisis to shoot a five under 66 at Valhalla.
On a day when Shane Lowry fired a three under 68,wearing odd socks for superstitious reasons having begun the trend at The Open, McIlroy ran up a double-bogey seven on the tenth after hooking his second shot out of bounds and then three-putted the 11th before responding with four birdies on the spin and another on the 18th.
"I was really annoyed," said McIlroy, who might have frittered a few more shots away six months ago had he suffered a similar blip. Not these days. "The second shot on ten was actually a bit of a shock, it's the worst shot I have hit the last few weeks.
"It came out of the blue and knocked me off track but to come back the way I did was great. I really needed that. It could have been a couple better but if those last few putts had dropped but 66 is a solid start."
McIlroy's long range eagle putt at the last slipped past the edge but it was another hugely impressive display by the 25-year old, who is chasing his fourth major and his third tournament win in a row with more mental resolve that he has displayed in his career to date.
He hit every fairway on the back nine, as well as his final eight greens in regulation with three of his birdies coming from inside six feet.
"To come back the way I did ... to continue that run, it was great," McIlroy said after laughing on TV when they flashed up his impressive driving statistics — 85% accuracy and an average drive of 327 yards.
"I really needed that. It could have been a couple better, but 66 the first day, it's a solid start."
After firing eight birdies, McIlroy is only one behind leaders Lee Westwood, Kevin Chappell and Ryan Palmer, who all shot 65.
“I know where my game is and I’m very confident with it," McIlroy said. "I’ve got a good thing going now and I’m just trying to ride the momentum.”
On his approach to the 10th, which hit a cart path and bounded over a fence, he said: "Things like 10 happen. You are going to hit bad shots every now and again. I really annoyed myself to bogey the next hole as well and not shake that off.
"After I hit that second shot on 12 I kind of gave myself a little, 'Come on', that was better. And that sort of kickstarted things. On 13, it was a good yardage for me, a good sand wedge and I could pitch it past the hole and the slope and the spin would help it come back.
"It nearly went in but I settled for a good birdie there. It was one of the good birdie opportunities on the back nine.
Having started all his major wins with a low round, McIlroy was more than pleased with his score, despite the mistakes at the 10th and 11th.
"If anything I am still slightly annoyed with myself for three-putting 11 after the double on 10," he said. "But my response after that was really good and just shows where I am mentally with my game. I am really happy with the five under start."
Referring to his driving statistics and the fact that he's never hit the ball better of the tee, he smiled and said on Sky Sports: "It doesn't get much better than that. I am driving the ball fantastically well. I am very comfortable with it and the fairways are a little more generous that they were at Firestone.
"I feel like I can stand up on these tees and hit it just as hard and feel like I can hit the fairways. If I can keep stats like that up for three days, I'll be happy."
Lowry could also be happy with his opening 68 which makes his 12 under par for his last three rounds of major championship golf following a 70-65 weekend that brought him a share of ninth in The Open.
Little wonder, after wearing odd socks at Hoylake, that he kept up the tradition in Louisville, following a bogey at his fifth hole with a hat-trick of birdies around the turn followed by two more at the fourth and seventh after another mistake at the 500-yard, par-four second.
If it was a good day for McIlroy, Lowry and European Ryder Cup Paul McGinley as Henrik Stenson, Edoardo Molinari and Chris Wood also shot 66, Mikko Ilonen a 67 and Ian Poulter and Joost Luiten 68s, it as a bad day for the US captain Tom Watson.
Tiger Woods struggled to an opening 74, while Matt Kuchar - who has already secured his place on the team for Gleneagles - withdrew before play began with a back injury and defending champion Jason Dufner pulled out with neck problems after ten holes.
“That wasn’t very good,” said Woods, who made only one birdie — a chip in. “A lot of bad shots. “I didn’t play as well as I wanted to. I didn’t get a putt to the hole,” he said. “That’s not a good combo.”
Displaying scant regard for the sea of red numbers on the scoreboard, Woods added: "I just don’t see, on this golf course, guys going super low here.
“I’ve got to get to under par by the end of tomorrow. ... If I can get under par for two rounds, that’s going to be right in the ballgame.”
Phil Mickelson rallied to shoot a 69 but the other member of the threeball, Pádraig Harrington, ended the day tied for 93rd with Graeme McDowell after a two over 73.
“I really didn’t get much out of it,” Harrington told the Irish Examiner before heading off to the practice green for a lenghty putting session. “Probably up until the last hole, very much every hole, was the worst I could’ve done at that hole.
“So it’s a little bit disappointing. A number of holes, I played them half-decently and ended up as bad as I could’ve done on them. I played well enough possibly to shoot two under par.”
McDowell was one under after four holes, having started at the 10th but bogeyed the 14th and 16th (three-putt) and double bogeyed the second after taking four to readch the green before getting a shot back at the seventh.
Understandably, he was in no mood to talk to the media afterwards.
Darren Clarke shot an eight over 79.