A hybrid swing drive system (1) of a hydraulic construction machine includes a variable displacement hydraulic swing pump (3) operable by a prime mover (2); a hydraulic swing motor (16) for performing a swing function of the machine; an accumulator (10); a controller (244); a swing control valve assembly (15) disposed in a first hydraulic path extending from the swing pump to the swing motor, the swing control valve assembly having a first position fluidly connecting the swing pump to a first side of the swing motor and a second position fluidly connecting the swing pump to a second side of the swing motor; and an accumulator control valve (12) having an open position fluidly connecting the accumulator to the first hydraulic path at an accumulator control valve connection point and a closed position fluidly isolating the accumulator from the first hydraulic path.