Kemar Roofe scored the only goal 55 minutes into the controversial first leg in the East Midlands to leave Leeds' destiny in their own hands.
As witnessed in the Champions League last week, and indeed in the other playoff semi-final between West Bromwich Albion and Aston Villa, a one-goal lead is hardly enough to be comfortable.
Leeds boss Marcelo Bielsa admitted as much after Saturday's match, pointing out that there are still 90 more minutes to play in what is a mouthwatering tie for the neutral.
United may hold the advantage thanks to Roofe's 15th goal of the season, but they will be without the striker on Wednesday because of a knock. That is a major blow for the Whites as Roofe has scored four goals in three games against Derby in 2018-19 - no player has scored more Championship goals against a single opponent this season.
Roofe's strike was not the biggest talking point of the first leg, though, as Derby had what they felt was a legitimate penalty overturned. Referee Craig Pawson pointed to the spot after Jack Harrison brought down Jayden Bogle, only for the officials to backtrack on their original call.
Derby also felt that Mateusz Klich could have been sent off after appearing to headbutt Fikayo Tomori in the chest, so there is no love lost between the sides, who were embroiled in the 'spy-gate' drama earlier in the campaign.
The bottom line ahead of this return match, however, is that Leeds hold all the cards. None of the previous 17 teams to have won the first leg of a second-tier playoff semi-final away from home has failed to progress to the final.
Finishing third in the table may also turn out to be a good omen for United because the team to have ended in that position have not lost the home leg of the playoff semi-final since 2009-10, when Nottingham Forest lost 4-3 to Blackpool. Third-placed teams have won seven of their eight home legs since then (W7 D1 L0).
Narrowly missing out on automatic promotion was a major blow for Leeds, who were let down by a couple of poor results on Easter weekend, but they are now well positioned to reach the playoff final. After 15 years away, they have never been so close to a Premier League return.
Recent form in Championship: WLLDLW
Derby finished the regular campaign strongly by winning three games out of four - and four out of six - to hold off Middlesbrough and Bristol City for sixth place.
Even taking their good form into account, however, and Leeds's loss of momentum in the closing fortnight, there was still a nine-point gap between the sides over the course of the campaign.
That told at times in the first leg as Derby failed to attempt an effort on target on home soil and had just two shots of any sort in the second half.
In fact, the Rams have managed just six shots on target and one goal across their three meetings with United this season, each ending in defeat. Twenty per cent of Derby's entire defeats in all competitions this season have been against Leeds United (three out of 15).
Frank Lampard needs to try something different in Wednesday's return match, then, especially when taking his side's away form into account.
The East Midlands outfit have won just one of their last 10 away matches in all competitions - 2-0 at Bristol City on April 27 - which does not bode well for this trip to Elland Road.
Lampard has urged his players to "be more confident" on Wednesday and put the home side under pressure. The Rams scored 69 goals during the regular season, averaging 1.5 goals a game, so they have the ability to at least draw back on level terms at Elland Road.
History will have to be made if they are to reach the playoff final again, however, with no side previously recovering from a first-leg loss on home soil at Championship level. Having got this far, Lampard will certainly not throw in the towel just yet.
Recent form in Championship: DWWDWL
Defender Pontus Jansson is still doubtful for Leeds, while Tyler Roberts, Barry Douglas, Ezgjan Alioski and Adam Forshaw are definitely out, with the latter being forced off the field in the first leg.
As a result of the mounting injury problems, teenage midfielder Jamie Shackleton could come in for just a fourth league start of the campaign.
Martyn Waghorn, who scored nine goals in the regular campaign, picked up an injury in training ahead of the first leg and is a major doubt for the clash in Yorkshire.
Leeds possible starting lineup:
Casilla; Ayling, Cooper, Jansson, Dallas; Hernandez, Klich, Phillips, Shackleton, Harrison; Bamford
Derby possible starting lineup:
Roos; Bogle, Keogh, Tomori, Malone; Mount, Johnson, Holmes; Wilson, Marriott, Lawrence
Head To Head
Leeds have beaten Derby County three times this season and are now aiming to beat a side four times in a single season for the first time in their history.
United won 4-1 at Pride Park in the second week of the season, with Roofe helping himself to a couple of goals. The striker also registered in his side's 2-0 victory in the most recent encounter at Elland Road four months ago, as well as scoring in Saturday's first leg.
Overall, the sides have met on 122 previous occasions - Derby winning 37 of those and losing 57, with their most recent victory coming five games ago in October 2017.
We say: Leeds 2-1 Derby (3-1 on aggregate)
Leeds have lost just two of their last 45 home matches in May in all competitions, although one of those was in the League One playoffs in 2008 when going down 2-1 to Carlisle United. The Whites did the hard work by coming away from Pride Park with a point and will now be confident of completing the job on home soil to keep that impressive run going.