Padraig Harrington led the four-man PGA Grand Slam of Golf after 10 holes at Port Royal in Bermuda.
At three under par the Dubliner, who was only called into the event on Saturday when Ernie Els withdrew injured, was one ahead of Masters champion Bubba Watson, two in front of US Open winner Webb Simpson and four clear of holder Keegan Bradley, himself a replacement for world number one Rory McIlroy.
Harrington's chance came after Graeme McDowell and Tiger Woods decided, like McIlroy, to play in Asia this week instead.
The Irishman followed a birdie on the long second with a bogey at the next, but then picked up further shots at the sixth, seventh and 10th.
Watson led after an eagle at the 517-yard seventh, but bogeyed the 10th, while Bradley was fighting back after standing four over after six holes.