Retief Goosen of South Africa fired a six-under-par 66 to take a one-shot lead over three-time defending champion Tiger Woods midway through the 36-hole Grand Slam of Golf in Kauai, Hawaii.
U.S. PGA Championship winner David Toms was in third place on 68.
The event sees the winners of this season's four majors play-off against each other.
David Duval, who collected his first major by winning the British Open in July, stumbled to a four-over-par 76.
Goosen won the U.S. Open after an 18-hole playoff with Mark Brooks at Southern Hills and lifted the World Cup trophy with fellow South African Ernie Els last Sunday.
Goosen has earned over $1.17 million in just nine U.S. PGA Tour starts this year and also won the Order of Merit on the European circuit.
Woods earned his place in the Grand Slam of Golf by virtue of his victory in the U.S. Masters at Augusta.