Léo Scienza and Taylor Harwood-Bellis scored within ten minutes of the hour mark to secure all three points for Southampton after they fell behind to a soft Carlton Morris goal just before half-time.
The Saints had to draw on the belief that has been building to overcome a stubborn and talented Derby County side, who have been on a good run themselves and still have an outside chance of sneaking into the play-offs.
Despite creating the better chances in the first half, Southampton could not take advantage, and when Finn Azaz gave away a very avoidable corner, Derby punished them.
Not for the first time this season, Southampton struggled to win the first ball from the ensuing corner. When it fell invitingly to Morris at the far post, he made no mistake and sent the ball crashing into the roof of the net.
After the break, Southampton regrouped and got themselves back on level terms when Scienza stepped inside his marker and sent a curling effort past Richard O'Donnell and just inside the far post.
After 64 minutes, Tonda Eckert made two changes that would ultimately settle the match in Southampton's favour. Kuryu Matsuki replaced Tom Fellows, and Cyle Larin replaced Ross Stewart.
A few minutes later, Southampton got their noses in front for the first time in the match. Taylor Harwood-Bellis headed home from a Ryan Manning free-kick to send the home fans into a frenzy.
Shea Charles and Samuel Edozie came on to freshen up the midfield with a quarter of an hour to go and Southampton protected their lead comfortably without really looking like extending it.
Player ratings: formation (4-2-3-1)
Goalkeeper and defenders
Goalkeeper - Daniel Peretz: 6/10 Southampton’s best goalkeeper in years has helped transform the club's fortunes. Given no chance by Carlton Morris' close-range effort.
Right back - James Bree: 7/10 The English full back has been brilliant since returning from his loan at Charlton, and suits the profile Tonda Eckert needs on the right-hand side. Fast becoming a fans' favourite. His passing was on point, and he got down the wing to support Fellows in attack.
Centre back - Taylor Harwood-Bellis: 8/10 THB made the most clearances in the match and scored the winner himself. Not a vintage display on the ball as Derby restricted his influence.
Centre-back - Jack Stephens: 7/10 Stephens was solid again and dealt with Derby's aerial threat well, making seven clearances. Celebrated like a captain should at the end of the match. He's put his early-season form to bed.
Left-back - Ryan Manning: 8/10 Manning had his best match in a while. He was strong defensively and accurate with his passing and crosses. His assist for Harwood-Bellis' winner was right on the money.
Midfielders
Centre midfield - Flynn Downes: 8/10 An absolute terrier in midfield. After it became obvious that the referee, Andrew Kitchen, was going to let the game flow, Downes needed no second invitation to get stuck in.
Centre midfield - Caspar Jander: 9/10 The German's best performance this season. Won the ball back in midfield repeatedly and drove the Saints forward through central areas. Justified his place in the starting lineup ahead of Shea Charles, and the interest in him from some of Europe's elite clubs.
Attackers
Attacking midfield (left) - Léo Scienza: 6/10 Despite scoring an exquisite equalising goal, Scienza was relatively subdued against Derby. He struggled to get down the outside of Joe Ward in the first half. What a goal though...
Attacking midfield (centre) - Finn Azaz: 6.5/10 Deducted a whole mark out of ten for giving away the corner that led to Derby's goal. Azaz was influential apart from that, but should have troubled the Derby keeper more with the four shots that he had.
Attacking midfield (right) - Tom Fellows: 6/10 Fellows probed throughout the match and stretched the play down the right often. But his final ball was poor again, and it was no surprise when he was replaced by Matsuki on the hour mark.
Striker - Ross Stewart: 6/10 Struggled against Max Johnston, who marshalled him well. Limited touches and zero attempts on goal.
Substitutes
Sam Edozie 6/10
Cyle Larin: 7/10
Kuryu Matsuki: 7/10
Shea Charles: 8/10
Nathan Wood: N/A