Kazeem Olaigbe solitary goal ensured Southampton's B team of three points at Snows stadium on Sunday.
Kazeem Olaigbe, who had a host of chances during the first half, gave Saints the lead with his first of the second; slotting home from close range on 48 minutes.
It was then about consolidating their lead, which the hosts did superbly and made it just the one defeat in the last four.
Saints’ line-up was once again a youthful one with the likes of Lucas Defise, Ethan Burnett and James Morris coming up from the under-18s side.
Caleb Watts and skipper Allan Tchaptchet provided first-team experience for Dave Horseman’s side.
The far more familiar Derby side put their hosts under considerable pressure during the opening moments, winning three corners before the game had hit the five-minute mark.
But Saints responded with a spell of pressure of their own; Sam Bellis, fresh off of his goal against Tottenham last time out, finding space on the edge of the box and forcing keeper George Sykes-Kenworthy into a strong save down to his left-hand-side.
The next fifteen minutes formed an end-to-end contest between two well matched sides, both endorsing a strong, closing down approach.
Both sides struggled to get an effort on each other’s goals due to the stubborn defences at both ends of the pitch.
But on 31 minutes Saints found their way through the Derby back-line; Defise’s through ball finding Bellis who, from a tight angle, forced another strong save from Sykes-Kenworthy.
The hosts were beginning to turn the screw, Jayden Smith the next to be played through before being tripped at the expense of a yellow card for Derby’s Jordan Brown.
Momentum was certainly tipping in Saints’ favour as the half wore on, Olaigbe next to have a go but was denied by a superb point-blank save on 40 minutes.
Olaigbe was involved once more in first-half stoppage time, keeping Sykes-Kenworthy busy till the end with a curling effort that looked destined for the top, right corner but for a fingertip save.
It was a backs against the wall job for the visitors as half-time loomed, Saints winning two corners in quick succession.
But at the break it remained 0-0 in what could be classified as an entertaining contest as far as that score line is concerned.
Derby looked to turn the tide with a half-time substitute, but Saints continued their dominance early into the second 45.
Finally, just three minutes into the second half, the hosts got their reward.
Olaigbe, who had been involved in the best of the Saints chances during the first half, followed in at the back post to slot home a deserved opener.
After his strike against West Ham it’s now two goals in three games for the young forward since the turn of the new year.
As expected Derby kicked into gear following the goal, winning a handful of free-kicks and corners as they pushed for an equaliser.
Cameron Cresswell produced a strong effort on the turn as County threatened from a corner, but it was sent behind with a strong hand by Jack Bycroft.
Bycroft was called into action once more minutes later, pushing away a close-range Harrison Soloman header in what was an impressive spell for the Saints keeper.
Moving forward the hosts continued to be a threat, Olaigbe and Defise carrying the ball forward with pace as Derby committed players up field.
As to be expected in a game of such tight margins both sides received their fair share of cautions during the second half.
But the stop-start nature of the fixture did not dampen the intensity, as both sides looked dangerous going forward.
With Derby closing in on Bycroft’s goal, Louie Watson was denied a point-black shot as a result of a perfectly timed David Agbontohoma challenge on 75 minutes.
Saints continued to produce some neat football to transition from defence into attack; substitute Goran Babic on his first appearance of the season for the B team linking well with Olaigbe and Defise on either flank.
But Derby kept pressing, Cresswell testing Bycroft again and forcing another strong save from the Saints keeper as the game entered the final ten minutes.
As the clock ticked down Saints the importance of the three points here grew, particularly after conceding late at home to Tottenham last-time-out.
Derby continued to peg back their hosts as the game went into stoppage time, but Saints stood firm.
The five minutes of stoppage time seemed to last an eternity, but the referee finally blew his whistle for the final time to confirm the three points.
It’s now one defeat in the last four for Saints, who will look to continue their good form with the visit of Manchester United next Saturday.