Busch led 143 laps and looked like he would ease to victory Friday night. But a late collision between Scott Legasse Jr. and Joe Nemechek brought out the race's 10th caution and set up a two-lap sprint to the finish.
That's when Busch's spotter noticed his right rear tire was flat, apparently because his car ran over debris. Busch tested the tire for several laps before rolling into the pits to loud cheers from Darlington fans.
Kenseth, who earlier set a track record to win the Southern 500 pole, still faced a green-white-checkered finish. That, too, went Kenseth's way when Morgan Shepherd spun out on the restart, ending the race.