Harrington has now won three majors in the space of 13 months
Padraig Harrington won his second straight major with victory in the 90th USPGA Championship at Oakland Hills.
The double Open champion held off Sergio Garcia and Ben Curtis on a dramatic last day to end two shots clear at three under after a 66.
Garcia (68) and Curtis (71) finished one under after a tense three-way tussle during the latter stages.
Colombia's Camilo Villegas and Swede Henrik Stenson ended one over, while Steve Flesch was sixth at two over.
World number two Phil Mickelson and Argentine Andres Romero were four over and Scotland's Alastair Forsyth, England's Justin Rose, South Korea's Charlie Wi and India's Jeev Milkha Singh were tied ninth at five over.
In the end it was a lot of sheer willpower that won it
The 36-year-old Harrington, who won his second straight Open title at Royal Birkdale last month, became the first European to win the USPGA since Scotland-born Tommy Armour in 1930.
Ireland's Harrington, who beat Garcia in a play-off for the Open at Carnoustie in 2007, also became the first player since Tiger Woods in 2006 to win back-to-back-majors - the American also won the Open and USPGA.
And he joined Woods (2006, 2000), Nick Price (1994) and Walter Hagen (1924) as the only players to win the final two majors in the same year.
"This is a different win," Harrington said. "I was comfortable with my game in winning the Open. This week I haven't been as confident and comfortable.
"Here I hit some of the wildest golf shots ever hit on a golf course on Friday. But I knew I had the ability to win if I got a chance. In the end it was a lot of sheer willpower (that won it)."
Harrington was three shots behind leader Curtis, the 2003 Open champion, at the start of the fourth round, but he joined Garcia at the top of the leaderboard on the 13th, sparking a tense finale, with Curtis still in contention in the final group.
Garcia led by one after the 15th but he found the water on the next hole and dropped a shot to rejoin Harrington and Curtis in a share of the lead.
Harrington, who holed a crucial 12ft par putt on 16, then drained another from eight feet for birdie on 17 to take a one-shot lead going down the last.
The only thing I can do is go back home with my head up high and keep working on it
Despite catching a fairway bunker with his drive, he conjured a magnificent par from 18 feet to Garcia's bogey.
"I think I was willing them into the hole at that stage," Harrington said. "You have to get focused and give it a go."
Curtis, who bogeyed the 17th, needed to hole his second shot on 18 to force a play-off but it proved too tough a task.
The 28-year-old Garcia is still searching for a maiden major victory and has now finished as runner-up three times. He did, at least, ensure his automatic selection for Nick Faldo's European Ryder Cup side.
"If a couple of things had gone differently, we would have been talking differently," said the Spaniard, who has had 14 top-10 finishes in majors. "But they went Padraig's way, not mine.
"The good thing about it is I felt good out there. I definitely feel like I played well enough to win but unfortunately it didn't happen. The only thing I can do is go back home with my head up high and keep working on it."
Last year Garcia was disqualified from the USPGA in Tulsa after signing an incorrect scorecard.
Curtis, who has won twice on the US PGA Tour since his Open triumph and was tied seventh at Royal Birkdale, also had the consolation of climbing into an automatic qualification place for a Ryder Cup debut in September.
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