Match ends, Southampton 2(3), Derby County 2(5).
Derby recovered from a two-goal deficit to edge past Southampton 5-3 on penalties in a thrilling FA Cup third-round replay at St Mary's.
Two goals in two minutes from Stuart Armstrong and Nathan Redmond had put the hosts on course for victory, after the visitors had earlier had a Craig Bryson goal disallowed by VAR.
But Harry Wilson's free-kick sparked a revival before Martyn Waghorn headed a late equaliser for Frank Lampard's side.
Redmond missed in the shootout for Southampton, allowing Rams captain Richard Keogh to strike the winning penalty.
Derby travel to League One Accrington in the fourth round on Saturday, 26 January (12:30 GMT).
Inseparable after 210 minutes
For the second successive meeting Derby displayed resilience to recover after falling two goals behind to Southampton, leaving the sides inseparable after 210 minutes of football.
In a game full of incident, it seemed the visitors had drawn first blood when Bryson produced a composed left-footed finish past Saint goalkeeper Angus Gunn in the first period.
But after a wait of two minutes, VAR judged forward Waghorn to be marginally offside before he played in the midfielder.
The introduction of Redmond helped Saints up the tempo after the break and they looked to have put the tie to bed with two goals in two minutes.
First Armstrong nodded in from close range to conclude a fine move involving Mohamed Elyounoussi and Shane Long. Jack Stephens then released Redmond who delivered a deft chip over Rams goalkeeper Kelle Roos.
Derby arrived on the south coast having lost eight of their past nine FA Cup games at Premier League opponents, but that statistic was clearly not on the players' minds as they surged forward to force a free-kick that Wilson curled home beyond Gunn - but only after more anxious moments following a VAR review.
And the battling visitors pulled level when Waghorn found the top corner with a well-placed header, forcing a goalless period of extra time before the successful shootout against last season's semi-finalists.
Derby's players showed great composure to convert all five of their penalties, with Southampton's only failure from 12 yards coming from the otherwise impressive Redmond, who sent an ugly effort wide of the mark.
'It's the FA Cup, it's magical' - what they said
Southampton manager Ralph Hasenhuttl speaking to BBC Sport: "Congratulations to Derby County. When they fight like they did in the two games, they deserve to go through to the next round.
"We didn't deserve to win. We had a few chances and didn't get the goal. We given the lead away two times so it was very disappointing. It was very frustrating. We were not very good in the first half so made changes to the system at half-time. Second half we were much better and played well for the two goals.
"But they got a goal from a set-piece and you could feel everybody thinking about the first game and getting a bit nervous. The opponent was in the right place to score a second time, and both sides had less power in extra time. We still had chances to win it but didn't take them."
Derby manager Frank Lampard on BBC One: "I'm delighted for the team and the squad. The workrate they put in there, having to come through VAR going against us in really small margins, they worked so hard and they were great penalties at the end and great courage from the lads.
"We've had two games against Southampton and a tough patch in the league, playing some really good teams, so the lads have been working so hard. It takes a lot to come back against Premier League opposition and show the fight and the bravery on the ball. I thought we were outstanding through the game. Some of the football in the first half was brilliant.
"It's a tough turnaround for the weekend and Accrington in the next round - it's the FA Cup, it's magical, look at the fans who have travelled. A big thank you to the fans. They're going to get back to Derby at whatever time and they're probably working tomorrow too so a big thanks to them because they helped us get over the line."
Wilson's free-kick threat - the stats
- Southampton have been eliminated from the third round of the FA Cup by a side from a lower division than themselves for the first time since 2001-02 when Rotherham United beat them 2-1.
- Derby County have now progressed from all three of their penalty shootouts in the FA Cup, all coming in third round replays (also against Sheffield Wednesday in 2007-08 and Millwall in 2009-10).
- Nathan Redmond has been directly involved in seven goals in his nine games under Ralph Hasenhuttl for Southampton in all competitions (five goals, two assists), as many as his last 63 appearances for the Saints combined before the Austrian's arrival.
- There were just 136 seconds between Stuart Armstrong opening the scoring and Nathan Redmond doubling Southampton's lead against Derby.
- Derby County's Harry Wilson has scored eight goals from outside the box in all competitions this season, at least twice as many as any other player in the top four tiers of English football in 2018-19. Five of those eight have been from direct free-kicks.
- Harry Wilson has had a hand in 14 goals across all competitions for Derby this season (11 goals, three assists), more than any of his team-mates.
- 66RamsaySubstituted forTargettat 110'minutes
- 2Cédric SoaresBooked at 75mins
- 51JohnsonBooked at 16minsSubstituted forRedmondat 45'minutes
- 17S ArmstrongSubstituted forSlatteryat 90'minutes
- 7LongSubstituted forGallagherat 73'minutes
- 37BogleSubstituted forHolmesat 94'minutes
- 44HuddlestoneSubstituted forLawrenceat 80'minutes
- 14MarriottSubstituted forNugentat 76'minutes
- 7WilsonSubstituted forBennettat 102'minutes
- Anthony Taylor
- Shots on Target
Penalty Shootout ends, Southampton 2(3), Derby County 2(5).
Goal! Southampton 2(3), Derby County 2(5). Richard Keogh (Derby County) converts the penalty with a right footed shot to the bottom right corner.
Goal! Southampton 2(3), Derby County 2(4). Matt Targett (Southampton) converts the penalty with a left footed shot to the centre of the goal.
Goal! Southampton 2(2), Derby County 2(4). Tom Lawrence (Derby County) converts the penalty with a right footed shot to the bottom left corner.
Goal! Southampton 2(2), Derby County 2(3). Jannik Vestergaard (Southampton) converts the penalty with a right footed shot to the bottom left corner.
Goal! Southampton 2(1), Derby County 2(3). Mason Mount (Derby County) converts the penalty with a right footed shot to the top left corner.
Penalty missed! Bad penalty by Nathan Redmond (Southampton) right footed shot is close, but misses to the right. Nathan Redmond should be disappointed.
Goal! Southampton 2(1), Derby County 2(2). David Nugent (Derby County) converts the penalty with a right footed shot to the bottom left corner.
Goal! Southampton 2(1), Derby County 2(1). James Ward-Prowse (Southampton) converts the penalty with a right footed shot to the bottom left corner.
Goal! Southampton 2, Derby County 2(1). Martyn Waghorn (Derby County) converts the penalty with a left footed shot to the top right corner.
Penalty Shootout begins Southampton 2, Derby County 2.
Second Half Extra Time ends, Southampton 2, Derby County 2.
Attempt saved. Mohamed Elyounoussi (Southampton) right footed shot from the left side of the box is saved in the bottom left corner. Assisted by Oriol Romeu.
Mason Bennett (Derby County) wins a free kick in the defensive half.
Foul by Oriol Romeu (Southampton).
Attempt saved. Martyn Waghorn (Derby County) left footed shot from outside the box is saved in the bottom right corner. Assisted by Kelle Roos.
Attempt missed. Jack Stephens (Southampton) header from the centre of the box is too high. Assisted by James Ward-Prowse with a cross following a corner.
Corner, Southampton. Conceded by Scott Malone.
Scott Malone (Derby County) wins a free kick on the left wing.