Kim shot a 1-under 69 at the East Lake Country Club to finish 36 holes of the final PGA Tour playoff event at 7-under 133.
That put him two shots in front of Sergio Garcia, who Kim defeated Sunday in the opening singles match of the Ryder Cup. Garcia shot a 65 Friday for a 135 total.
"I'm playing great right now, I love playing this game out here," Kim said. "It's hard for me not to have fun right now."
Phil Mickelson, who partnered with Kim in three matches during the American Ryder Cup victory, was alone in third at 136 and Camilo Villegas was fourth at 138. The only other players under par were K.J. Choi and Mike Weir at 139.
The Tour Championship was supposed to determine the winner of the FedEx Cup and its $10 million top prize, but Vijay Singh wrapped up that honor by winning two of the previous three playoff tournaments.
Singh, needing only to complete all four rounds to clinch the FedEx title, was tied for 23rd in the 30-player field at 147.