Iceland international Hreidarsson slammed the ball home from close range following David Cotterill's corner midway through the first half and helped marshal a Pompey defence that kept a sixth successive clean sheet.
But while Steve Cotterill's side climbed to within striking distance of the top six, Sheffield United appear to be in freefall under manager Micky Adams.
The former Port Vale chief is yet to taste victory in 13 matches since taking over from Gary Speed as Blades boss in December, and watched his charges slip to a third successive defeat at Fratton Park which leaves them six points adrift of safety.
Source: PA
Source: PA