Championship: Brighton and Hove Albion 1 Derby County 2
On-loan Manchester United forward Jesse Lingard gave the hosts a deserved 18th-minute lead but Chris Martin levelled from the penalty spot.
Martin's shot in first-half stoppage time then bounced in off unfortunate Brighton goalkeeper Tomasz Kuszczak to take Steve McClaren's team a step closer to a top-flight return.
Derby finished 13 points higher in the Championship table than Brighton, who needed a last-gasp Leandro Ulloa goal at Nottingham Forest on the final day to keep their promotion dream alive.
But it was the home team who swarmed forward in the opening stages on Thursday, with attacking right-back Inigo Calderon to the fore.
In the 11th minute the Spaniard was denied a penalty after tangling with Craig Forsyth but he was instrumental seven minutes later when Brighton went ahead.
Calderon broke into the Derby box and cut back to Lingard, who finished superbly at the second attempt after his initial shot was blocked by Jake Buxton.
Forsyth's involvement in another penalty incident - Matthew Upson rashly halting the full-back's storming run forward - helped to bring Derby level in the 29th minute, Martin converting with aplomb.
Derby showed their supreme threat on the counter attack through Jeff Kendrick and Johnny Russell either side of a headed chance for Ulloa in the 35th minute, and the visitors led thanks to a slice of luck in first-half stoppage time.
Russell teed-up Martin and the striker's rasping shot cannoned back off the crossbar to hit Kuszczak in midair and fall in.
Kuszczak diverted a close-range Martin attempt into the side netting as Derby went for the kill early in the second period
Opposite number Lee Grant produced a stunning 61st-minute save, tipping onto the bar after Ulloa miscued a volley into the turf, while the ex-Burnley man produced an athletic double save to deny Lingard and Stephen Ward as the clock wound down.
Brighton could not find a way through and must come from behind in Sunday's second leg to avoid a repeat of the semi-final heartache they suffered at the hands of bitter rivals Crystal Palace a year ago.