Rene Howe gave the Gulls a third-minute lead before a Mark Ellis own goal and Evans’ double sealed the game despite a late consolation from Danny Stevens.
After an emotional minute’s applause for former Blades player and manager Gary Speed, Martin Ling’s men raced ahead. Kevin Nicholson’s searching free-kick was flicked on by Lee Mansell and top-scorer Howe smashed the ball past Steve Simonsen at his near post.
The Blades almost levelled the tie on 25 minutes but Evans’ free-kick was spectacularly turned over by Bobby Olejnik. In-form Welsh striker Evans rattled the right-hand upright seven minutes after the restart before the comeback began.
Centre-half Ellis sliced Matthew Lowton’s low left-footed cross into his own net on 68 minutes before Evans lashed home from the edge of the area following a strong run.
Evans slotted in to seal the win 12 minutes from time following Olejnik’s poor clearance before Stevens powered home at the death.