Padraig Harrington didn't know he would be playing in the PGA Grand Slam of Golf until Saturday.

On Wednesday, he won the event.

After playoff losses in 2007 and 2008, the Irishman shot a 4-under 67 on Wednesday to win by a single stroke at Port Royal Golf Course.

Harrington, who replaced Open Champion Ernie Els, finished at 9-under 133 and won by a single stroke over U.S. Open winner Webb Simpson, who fired a 6-under 65 in the second and final round.

This exclusive event is reserved for the season's four major champions, but Els withdrew and was replaced by Harrington.

Keegan Bradley, the defending champion, who got into the field when PGA Champion Rory McIlroy pulled out, posted a 4-under 67 and matched Masters winner Bubba Watson, who only managed an even-par 71 on Wednesday, in third at minus-3. Bradley holed out from 129 yards away for an eagle on the par-4, 14th.

Harrington, who won the Open in 2007 and 2008, then added the PGA Championship later in the summer of 2008, started the final round with a 2-shot lead over Watson, but it came down to Harrington and Simpson.

Harrington parred his first four and Simpson birdied the par-3 third to get within two. Both players birdied the fifth and Simpson cut the margin to a stroke with a birdie at the seventh.

The pair both birdied the par-3 eighth, then Harrington essentially put the tournament out of reach after the turn.

Harrington birdied three in a row from the 11th to get to 10-under par. Simpson birdied 14 and 17, but was two behind thanks, in part, to Harrington's four consecutive pars from the 14th.

Harrington held a 2-shot lead on the 18th and bogeyed the hole. All it meant was his margin of victory was a stroke.

Simpson was the lone player in the field to post a bogey free round over the two-day event. "I'm really encouraged with how I played today," said Simpson. Wish I could have probably had that shot back on 18, the iron shot. I just came out of it. But overall, good day, and you know, it's been a great week."

NOTES: Harrington pocketed $600,000 for the victory, while Simpson earned $300,000 and Bradley and Watson both received $225,000.