Cometh the hour, cometh the man. The real Padraig Harrington is finally back.

Just when it looked as though he was about to slink off to the range to work himself out of a slump that has seen him miss eight cuts from 16 strokeplay starts this season and slip from third to 17th in the world, the Dubliner raced two strokes clear of the field when he opened with a sensational, six-under par 64 in the WGC-Bridgestone Invitational at Firestone Country Club.

Never better than 12th here - a result he recorded on his debut in 1999 - Ireland's triple major winner finally showed his true colours when he blasted six birdies in an immaculate performance that will be a massive boost to his confidence ahead of his US PGA Championship defence at Hazeltine National next week.

It was Harrington’s lowest ever round at his venue - a stroke better than the five under par 65 he recorded in the second round in 2002, when he finished tied for 27th.

More significantly, it was his best round of the year by two clear strokes and left him two clear of Thailand's Prayad Markasaeng, South Africa's Tim Clark and American Scott Verplank.

True, he hit jut seven of 14 fairways but his short game was back to his magical best, especially the putter. He holed three putts over 20 feet on three of his last four holes.

“I'm definitely happy with the score,” said Harrington, who defends his US PGA title next week. “This is the most I can remember getting out of a round since the US PGA last year.

“I walked off the golf course feeling like I got a couple more shots. And I feel good about the fact that I got one or two more than I probably should have. If I shot 66, I would have said, well, that was about right. A 64 is a little bonus.”

Still,the Dubliner refused to get carried away by his first really positive round of golf in months.

“It's just a round of golf. It doesn't change the last six months, it won't change the next six months,” Harrington said.

Harrington's round was one of just two bogey free rounds recorded with the other coming from Verplank.

"I think that's one of the best rounds I've seen him play. He really played well," Verplank said. "I know he's struggled a little bit and he's probably gotten a little too much criticism for working on his game, but you know what, he's the only one that knows what he needs to do. Regardless of what anybody else says, he's the only one that knows. If today is any indication, then he's doing something right."

Steve Stricker, Ian Poulter and Zach Johnson are three shots behind Harrington after 67s with Tiger Woods and Sergio lurking just four behind after a 68.

But while there was general excitement all round at Harrington’s renaissance, the man himself was determined to stay on an even keel.

“It's just a round of golf. It doesn't change the last six months, it won't change the next six months,” Harrington said.

Starting on the back nine on the famed South Course, Harrington began with a birdie at the 10th, firing a wedge to eight feet for a perfect start and then showed that the hard work he has done on his short game recently has not been for naught.

Having saved par from around six feet at the 11th and 13th to remain on one under par, he moved up a gear around the turn.

A birdie at the par-five 16th, where laid up 93 yards from the water protected green and holed a slick 10 footer down the hill, was quickly followed by two more in quick succession and a great par save.

At the first, he wedged to 12 feet off a perfect drive and then two-putted from 25 feet at the 526-yard second to draw level with clubhouse leaders Marksaeng and Clark, who had opened with four under par 66’s in perfect conditions.

Then at the tough, 469 yard sixth, he flashed a greenside bunker shot 21 feet past the hole but rammed home the par saver to remain at the top of the leaderboard. The familiar smile was in evidence again.

Having survived a scare there, Harrington hit a 227-yard hybrid to 26 feet at the par-three seventh and drained the putt to move clear at the top. But there was still more magic to come.

At the eighth his 143 yard approach finished 23 feet away but he waved his putter like a wand once more and drained that one too to move to six under par.

While all this was going on, Tiger Woods opened his bid for a seventh win in this event with an two under par 68 while world number two Phil Mickelson produced three back nine birdies to card a level par 70 on his return to the tour after a six week absence to help his wife Amy and mother Mary in their battles with breast cancer.

The 38 year old Californian was rusty on his front nine, where he ran up a double bogey six at the 11th, taking four to get down from no more than 20 feet from the pin.

He was three over par through the turn, but somehow manage to bounce back with three birdies on his homeward nine and remain on the fringes of contention.

Reflecting on his lay off, Mickelson said: “I missed playing golf, and I missed competing. I missed both of those aspects, and in the off‑season I enjoyed taking that time off, I enjoyed being away from the game. This was different.

“I think that I've just been playing golf as a kid ever since I was a little kid. I've just loved playing golf. I've always loved competing, whether it was for a soda, a golf ball, tees, or on the PGA TOUR for huge purses. I missed the competition. I also just miss being on the golf course. It's where I've grown up, and I just love this game of golf.”

Just 79 players will tee it up in today’s second round following the withdrawal with a rib injury of England’s Paul Casey, the world number three, after just six holes.

“I think I strained an intracostal,” said Casey, who was level par at the time. “I actually did it a couple of weeks ago, a week before the Open, hitting drivers with my coach.”

Almost certainly a non-starter for next week’s US PGA, Casey added: “I've got to be careful because there's a lot of important golf left - FedExCup, Race to Dubai, a lot of big stuff.”

Darren Clarke, the 2003 champion, opened with a respectable, one over par 71.