Manning opened the overtime with a jumper and Washington's Brent Price answered with a 3-pointer.
The Bullets got the rebound and Brent Price nailed two free throws for a 4-point edge with 28 seconds remaining.
Brent Price had 16 points, while his brother Mark, playing his fifth game back from a foot injury, had 13.
Brent Price scored 30 points, a record for him with the Bullets.
Then Rumeal Robinson (12 points) scored on a drive to the basket after a reverse layup by Washington's Brent Price.
Brent Price will be out at least two months with an arm injury.
John Hudson scored 19 points and Brent Price 18 for South Carolina (5-0).
Brent Price then missed a 3-pointer in the final seconds.
He was fouled by Brent Price and converted the free throw.
He had made 12 3-pointers in a row entering the game, needing just 1 to tie the league mark set by Brent Price.