Late replacement Padraig Harrington fired a 5-under-par 66 at the Port Royal Golf Course on Tuesday to lead the PGA Grand Slam of Golf after one round.

This two-day, 36-hole tournament traditionally features the winners of the four major championships.

Harrington is filling in for British Open winner Ernie Els, who withdrew on Saturday due to a left ankle injury. Masters Champion Bubba Watson and U.S. Open Champion Webb Simpson are also in field, as is the Grand Slam's defending champ Keegan Bradley -- a replacement for PGA Championship winner and World No. 1 Rory McIlroy.

Watson sits in second place at 3-under 68, Simpson is a stroke back in third place and Bradley rounds out the field at 1-over 72.

An eagle on the par-5 seventh helped Watson to a 1-stroke lead at the turn, but the Irishman Harrington pulled away on the outward nine.

Harrington, who was named to the field after second and third alternates Graeme McDowell and Tiger Woods declined invitations, opened the back with one of his seven birdies, then turned the trick in succession on 14 and 15 to grab a 3-stroke lead over Watson and Simpson.

After bogeying the par-3 16th, Harrington closed with a birdie and a par, respectively, to wrap up his 66.

On Wednesday, Harrington, the lone non-American in the field, will attempt to secure his first career Grand Slam of Golf victory. The three-time major winner lost in a playoff in his two previous tournament appearances.

NOTES: Harrington fell to Angel Cabrera at this event in 2007 and then to Jim Furyk in 2008...Els won the Grand Slam of Golf in 1997 and 2010...McIlroy had already committed to the European Tour's BMW Masters in Shanghai, China...The final round begins Wednesday at 8:30 a.m. (ET).