Subs Jamie Jackson and John Johnston turned the game on its head, although the latter blotted his copybook by being sent off in added time for a dangerous tackle after scoring a sensational winner.
The Reds had played well for much of the first half, only to lose their way as Elliott Durrell and Danny Newton established what looked to be an unassailable advantage for the Lambs, who had been unbeaten in all 11 League games so far this season.
And had Shane Clarke taken any of his three clear chances the margin would surely have been insurmountable for the visitors, but perseverance was key and determination won the day.
Early Alfreton pressure had seen Cliff Moyo’s cross helped into the box by Robertson with Dan Preston hacking clear, then Belshaw fumbled a Leesley cross under pressure with five minutes elapsed.
Todd Jordan’s header from a Leesley corner lacked power, Ryan Wilson blazed over after a succession of long throws by Niall Heaton, and Heaton drew a flying save from Belshaw before Tamworth had even threatened the Reds.
Duke dealt with Newton’s 25th minute strike, and as the hosts grew in confidence both Clarke and Ross Dyer went close in the minutes leading up to their opener.
Nine minutes later they secured their first corner which led to the opening goal of the tie, Durrell’s long-range shot taking a wicked deflection off Tom Allan to flash past Matt Duke.
Alfreton were pushed back on the defensive for the majority of the second half, Duke having to make a point-blank save from Clarke two minutes after the break, and it certainly was not a shock when Newton headed their second goal from another Jed Davies corner in the 75th minute.
Crucially though the Reds responded immediately, with Jackson running on to an Andi Thanoj through-ball to reduce the deficit with a firm drive after entering the area, and their followers were ecstatic on 89 minutes when Newton needlessly handled a Joe Leesley free-kick under pressure from Sam Berry.
Up stepped Jordan Robertson to arrow the ensuing penalty low to keeper James Belshaw’s right, and Alfreton were level at 2-2 with plans taking place for Tuesday night’s replay.
Then with one minute of added time elapsed Johnston latched on to a cross from the left to smash the ball home from the edge of the area, but moments after the restart a rash two-footed tackle on Louis Ezewele left the match-winner with a red card against his name.
ALFRETON: Duke; Moyo, Allan, Jordan, Berry, Heaton, Wilson (Thanoj 38), Leesley, Jones (Johnston 67), lronside (Jackson 67), Robertson.
Subs not used: Smith, Draper.