Justin Edinburgh's side thoroughly deserved another chance against Coca-Cola League One strugglers Brighton & Hove Albion.
But Liam Dickinson struck for the second time to send the Seagulls through.
Diamonds made a nightmare start as a mix-up b
etween keeper Nathan Abbey and skipper Jamie Stuart gifted the opening goal to Dickinson after only two minutes.
Neil Cousins was carried off but Lee Tomlin produced a stunning equaliser with a low finish past Graeme Smith.
Nicky Forster wasted a couple of clear-cut chances before making it 2-1 from the penalty spot following Curtis Osano's trip on the veteran striker.
Diamonds got back on level terms as Aaron O'Connor cut inside and found the far corner.
The second half was much quieter but Forster missed the best opportunity by firing wide.
But with five minutes to go, a challenge on Forster saw the ball break kindly for Dickinson to beat Abbey again.
Brighton: Smith; Hoyte, Virgo, Elphick, El-Abd; Bennett, Navarro (sub Dicker, 60 mins), Crofts, McLeod (sub Hart, 50 mins); Dickinson, Forster. Subs not used: Wright, Cox, Tunnicliffe, Davies, Pelling.
Diamonds: Abbey; Downer, Osano, Stuart, Robinson; Cousins (sub Pattison, 6 mins; sub Akurang, 41 mins), Byrne, Porter, Tomlin; Farrell, O'Connor (sub McNamara, 78 mins). Subs not used: Roberts, Corcoran, Smith, Ainsley.
Referee: Phil Gibbs.
Goals: Dickinson (2 mins, 1-0); Tomlin (10 mins, 1-1); Forster pen (18 mins, 2-1); O'Connor (34 mins, 2-2); Dickinson (85 mins, 3-2).
Bookings: Porter, Osano, Stuart, El-Abd (fouls).
Attendance: 3,638 (261 away fans).
