Padraig Harrington was right on Tiger Woods' tail after sparkling first round duel left them at the top of the field in the 91st US PGA Championship at Hazeltine National.

In a class of their own: Woods and HarringtonJust four days after losing the Bridgestone Invitational to Woods in an action-packed final round battle, the Dubliner gave the world No 1 fair warning that he is not about to give up the Wanamaker Trophy without a fight.

Ireland’s triple major winner shot a four under 68 to finish the day just a stroke behind Woods, who had five birdies in an immaculate 67 that was good enough for the championship lead and the perfect start to his bid for a record-equalling fifth US PGA title.

Three-time major winner Vijay Singh, Australia's Robert Allenby and Matthew Goggin, Americans Hunter Mahan and David Toms and Spain's Alvaro Quiros shot three under 69s to share third place.

But Woods and Harrington showed that they are in a class of their own right now and while last night’s action lacked the intensity that marked last Sunday’s showdown at Firestone, they matched each other shot for shot before the American separated himself with a late birdie to take the first round lead.

There were smiles galore during the round but Harrington knows that things will get deadly serious if they are in the mix come the back nine on Sunday and after mixing five birdies with just one bogey, he was inspired to equal his best opening round in a major and keep the Woods well and truly in his sights.

“When you play with Tiger, it pushes you and that’s what I like," said Harrington, who confessed that he felt so physically drained after last week's battle in Akron that he almost fainted after his Tuesday morning press conference. “You have to go to a new level. There is no point in being cautious or playing safe and I get sometimes get caught in the middle. You either go for it or you don’t.

“You either play well or you don’t is my attitude and Tiger brings that out. He pushes you a little bit more and you have to go for your shots if you want to compete.

"I think there's a factor about the fact that Tiger is five‑under par and looks like he's playing well and looks like he could move away and the key will be obviously if he's moving away to make sure I'm moving away with him.

"I don't want to give him a three‑shot start on Sunday but obviously I'm very happy. First day of a major it's always good to keep yourself in there. I think I probably did a little bit more than that. Anytime you break 70 you've got to be very pleased.”

With 2002 champion Rich Beem going almost unnoticed as he fired a 71, Harrington and Woods looked totally at ease as they flashed smiles and exchanged grins on the way round.

What better example than their visit to the uphill, 606 yard 11th, which was out of range for both men yesterday but not for the big-hitting Spaniard Quiros.

After laying up, Harrington missed the green with his third but chopped out superbly to three feet from greenside rough and was shaping up to hit his par putt when Quiros’ ball scuttled onto the front edge of the green - a driver from the deck that found the putting surface 290 yards away.

“There couldn't be anybody playing the game competitively who could have knocked it up there on 11 today,” Harrington said. “It was phenomenal. It was a serious hit. We were all greatly amused by the fact that there was a player out there who can actually hit a ball like that.”

The Spaniard held his hand up in apology and headed straight towards the 12th tee to say sorry in person, getting the thumbs up from a smiling Tiger.

But Woods saw the funny side, explaining: “There was nothing to apologise for. I mean, that's just stupid long, isn't it? To hit it that far into the wind is phenomenal. It's just absolutely phenomenal. I used to be able to move the ball, not anymore. I just plod my way around, shoot 67.”

With Rich Beem an almost anonymous member of the threeball, the 501 yard 12th offered a perfect cameo of their growing rivalry.

Harrington was the longer of the three off the tee and after watching Beem come up short with a career best rescue woods and Woods fire a long iron 20 feet past the flag, the Dubliner rifled a superb five iron to just eight feet.

Almost inevitably, Woods holed his putt for birdie but Harrington followed him in to get into red figures and never looked back.

Starting on the back nine, Harrington was level with Woods on two under through the turn before the continued their duel on the front nine.

Harrington bogeyed the first after a poor bunker shot but then matched Woods with birdies at the second (four feet) and third (pitch to three feet) before firing a wedge to six feet at the sixth to draw level with Woods on four under.

Woods edged their head to head by a shot when he birdied the par-five seventh from three and Harrington believes the world No 1 is more in control of his game than ever.

Having taken time out to watch his rival, Harrington said: “Tiger never looked like not breaking 70 today. He is not as flamboyant as he was and become more conservative but he is very much in control.

“But my focus was good and all I have to do now is go and work on bunker shots. I missed two today.”

Watched by a massive gallery, Harrington and Wood duelled it out right form the start with the Dubliner going with a driver off the 10th tee so he could attack the flag.

Harrington’s approach sailed straight over the pin and after watching Woods lip out from 20 feet, he two putted from a shade closer for a solid opening par.

After those two tremendous birdies at the 12th, Woods edge ahead again at the 642 yard 15th, which playing downwind. While both men found greenside sand in two with three woods, Harrington’s attempted escape ran 18 feet past and as he failed to get up and down for his birdie, Woods splashed out stone dead to sneak one stroke in front on two under.

Harrington looked totally focussed on the job in hand, however, and got that shot back at the signature 16th down by Lake Hazeltine, where he fired an eight iron to eight feet and rolled into the putt.

Beem birdied the 16th, 17th and 18th to leave all three tied on two under heading through the turn. But he faded to a 71 in the end as Harrington and Woods continued to rack up the birdies in a battle that could continue for another three days.

That could depend on how Harrington is feeling physically after confessing that he was physically drained by his experience in Firestone, where he took an eight at the 16th to hand Woods the title.

“I had to leave the golf course on Tuesday because I was so tired,” Harrington said. “I was feeling awful. After I did my press conference, I felt like I was going to faint.

“So my preparation has been poor. So most of my preparation this week has been about just getting my mind‑set right and I played nine holes on Monday and 18 holes on Wednesday, which is probably less than I would normally.

“I hardly spent any time hitting bunker shots and putts, which it cost me today. I hit two good bunker shots today. But both of them, obviously the ball doesn't spin much out of the sand here. And I hadn't realised that.”

World number two Phil Mickelson struggled badly, carding an two over par 74 that leaves him in danger of missing the cut.