SOUTHAMPTON, Bermuda, Oct 23, 2012 (AFP) - The four-man, 36-hole showdown for a $600,000 top prize at Bermuda's Port Royal Golf Course was designed to feature the year's four major champions.
But three-time major champion Harrington replaced British Open winner Ernie Els, the South African bowing out with a left ankle sprain, to become the only non-American in this year's field.
Northern Ireland's Rory McIlroy, the PGA Championship winner, withdrew to play a European Tour event in China, being replaced by defending event champion Keegan Bradley, the 2011 PGA Championship winner.
Reigning US Open champion Webb Simpson was third on 69 with Bradley fourth on 72 entering Wednesday's final round.
Harrington birdied the par-5 second but took a bogey at the par-3 third. Birdies at the sixth and par-3 seventh and another to begin the back nine put the Irishman in the lead.
Back-to-back birdies at 14 and 15 stretched Harrington's lead and he answered a bogey at the par-3 16th with a birdie at the par-5 17th.
"It was nice to get off to a good start," Harrington said. "Hopefully I will be a little bit sharper with my wedges tomorrow and I'll hole a few more putts."
Harrington lost the Grand Slam in a playoff to Argentina's Angel Cabrera in 2007 and to Jim Furyk in a 2008 playoff.
Watson led early thanks to an eagle at the par-5 seventh but he made bogeys on three of the first four holes on the back nine to stumble back to 68 at the finish.
Simpson kept pace with Watson before a bogey at 18 dropped him three adrift of Harrington while Bradley made bogey on two of the last three holes and never recovered from a double bogey at the fourth and bogeys at the fifth and sixth.
"I've been six shots back before with less time, so I look forward to going out tomorrow and just making a million birdies and having a good time," Bradley said.