Steve Elkington - play-off victory. (Allsport)

Tiger Woods emerged triumphant from one of the most thrilling championships in years tonight - and with it became only the second man in golfing history to win three majors in one season.

A drama-packed final round in the United States PGA championship in Louisville saw Woods and fellow Californian Bob May both break the event's record by finishing 18 under par - Woods after shooting a 67 and the unfancied, but inspired May his third successive 66.

For the first time since the tournament switched from match play to stroke play in 1958 the title was decided over three holes of stroke play rather than sudden death.

Back they went to the 16th tee and Woods, who had won the US Open and Open this summer in starkly different circumstances by 15 and eight strokes, brought huge roars from the crowds ringing the green when he made a 20-footer for his third successive birdie and a one-stroke lead - after May had almost holed his pitch-and-run.

May, a 31-year-old playing only the fourth major of his career and without a single US Tour win, and Woods both scrambled pars at the 17th as tension showed in both men.

But that had nothing on the long 18th, where an hour earlier May had holed a 15-footer for birdie and Woods had followed him in from six to force the extra holes.

Woods wildly hooked left and the ball appeared destined for bushes until it appeared - after a spectator had raced to the spot - bouncing down a cart path.

May drove into deep rough, both were still in rough after two, then Woods's third found sand. May was on in three, but on the wrong level.

Unless May holed Woods had to get up and down to match Ben Hogan's 1953 feat - and he splashed out to within inches of the cup.

May gave his putt a superb run at the hole, but it bent across the front and just missed.

By holing his tap-in Woods also became the first man since Densmore Shute in 1937 to make a successful defence of the trophy.

Dane Thomas Bjorn finished third, but was five behind Woods and May after a closing 68, while Jose Maria Olazabal, after his major championship record-equalling 63 in the third round, bogeyed the par five last to drop back into a tie for fourth with Australians Greg Chalmers and Stuart Appleby.

Darren Clarke was top Briton in a tie for ninth.

One ahead overnight on 13 under par, Woods struggled over the opening six holes and ended them trailing by two.

His problems had begun with a bogey six at the second. His third shot, a chip from tangly rough, ran over the green, his chip back ran four feet past the hole and he pulled the putt.

With May getting up and down from a bunker, the two changed places at the top of the leaderboard.

May, in rough off the fourth tee, conjured up a magical approach to three feet to go two ahead and Woods missed a real chance to strike back at the 421-yard sixth when he three-putted after May had failed to get up and down from just over the green.

But Woods hit back by pitching to within a foot of the hole for a birdie at the long seventh and made a 12-foot putt on the 166-yard eighth to level.

And even when May got up and down from sand at the 551-yard 10th to move to 14 under, Woods, also bunkered, followed him in for a matching birdie four.

But he could not match May's 30-foot putt on the short 11th and so with seven to play it was the underdog leading golf's top dog.

Woods's character came shining through at the 467-yard 12th and short 14th. Each time he holed from 14 feet for birdie after May had struck his irons to three and six feet respectively.

To his eternal credit, May followed him each time to stay in front.

May then appeared to have a chance to go three clear with three to play when he hit his approach to five feet on the 402-yard 15th and Woods missed the green.

But the holder, after a poor putt up the bank onto the green, made a 10-footer for his four and May missed to remain only one in front.

Both parred the 16th, May recovering well from driving into the rough, but then the championship came to its dramatic conclusion.

Woods drew level with a massive drive and 96-yard pitch to three feet on the 17th, then bravely holed on the last after May's magical 15-footer.

The play-off had a hard act to follow, but it managed it. And Woods managed another slice of history.

Clarke and Lee Westwood, second and first on the European money list, both shot 67, but had left themselves far too much ground to make up and finished nine under and eight under respectively.

Clarke had threatened to make it a real week to remember by opening with a 68, but followed with two 72s and the only consolation of his finish - apart from the money, of course - was that he has closed the gap on his close friend at the top of the Order of Merit.

``This game has me baffled,'' said the Ulsterman. ``On the driving range on the first day I didn't know what was going to happen, then in the middle two rounds I thought I was going to shoot the lights out, then today was the same as the first day.''

Westwood, after a round that began at lightning pace with an eagle at the second and birdie at the fourth, said: ``I squandered too many shots, but on the positive side it was probably my best ball-striking major.

``I missed three times from two feet yesterday, twice today - and in the first round the longest putt I holed was four feet. No wonder I'm bottom of the putting stats.''

The rest of their Ryder Cup team-mates were down in the pack, with Colin Montgomerie having his second successive 70 for two under, Nick Faldo (73) and Andrew Coltart (71) finished one over, Padraig Harrington (74) two over and Paul Lawrie (75) six over.

Montgomerie made one of his best up and downs of the week at the last and, having not wanted to speak after two of his three earlier rounds, was in much more talkative mood.

``I had fun out there,'' he said. ``For the first time in a number of years I've enjoyed it over here. There's been a big change in the way the American crowd has accepted and respected me.

``Unfortunately, the golf didn't work out - I didn't chip and putt well enough. But forgetting the golf, I think it's a big step I've taken.''

He hopes for more of the same in this week's $3million NEC world championship in Akron, Ohio - and obviously more success too.