Newcastle United hold a slender advantage from the Carabao Cup semi-final first leg as Joelinton's goal gave them victory at Southampton.
The Brazilian made amends for missing an earlier easy opportunity when he arrived on the end of substitute Alexander Isak's perfect low cross to make the decisive contribution with 17 minutes left.
Newcastle will now be favourites to reach their first major Wembley final since the FA Cup in 1999 as they attempt to end a trophy drought stretching back to the 1969 Inter-Cities Fairs Cup win.
And if Newcastle do complete the job in the second leg on Tyneside they will owe a debt of gratitude to Nick Pope, who made two vital second-half saves from Southampton substitute Che Adams with the score still goalless, extending his remarkable personal record to 10 successive clean sheets.
Southampton, who had what they thought was an equaliser from Adam Armstrong disallowed for handball by VAR, saw their night of frustration complete when defender Duje Caleta-Car was sent off in the closing minutes for a second yellow card.
Joelinton was a central figure, seeing a first-half goal contentiously ruled out for handball then blazing over an open goal when it seemed easier to score, before hitting the target to make Eddie Howe's side firm favourites to reach Wembley.
The two sides meet at St James' Park in the second leg on Tuesday, 31 January.
Newcastle's march continues
Newcastle United's dream season under Howe now carries the real prospect of a Wembley appearance to go alongside their challenge for a place in the Premier League's top four and next season's Champions League.
They needed the outstanding interventions of keeper Pope to keep Southampton at bay when they threatened after the break but Newcastle always carried a threat, even if much of their finishing was wayward, and eventually sharp work by substitute Isak delivered a chance Joelinton could not miss.
Newcastle's defence is so mean-spirited this season that one goal is invariably enough and so it proved as, for all Southampton's effort, they simply could not break through, leaving the Toon Army packed into a corner of St Mary's celebrating at the final whistle.
Howe will know Newcastle were not at their best and showed nerves at times, but there will be satisfaction at getting this part of the job done against a Southampton side that disposed of favourites Manchester City here in the quarter-final.
It leaves Howe's side in a favourable position for the second leg in front an expected capacity crowd at St James' Park, with a Wembley final against either Manchester United or Nottingham Forest as the prize on offer.
Southampton frustrated
The hosts were left cursing both Newcastle's outstanding keeper Pope and VAR as they face an uphill task to overturn this slender deficit in their bid to reach Wembley.
Saints' manager Nathan Jones was locked in deep discussion with referee Stuart Attwell as they left the field, discussing the VAR decision that ruled it was a handball by Adam Armstrong as he bundled home what he thought was the leveller just after Joelinton's goal.
Jones, however, will also know the key chance fell to Adams when he raced clear with the game in the balance only to be thwarted by the outstretched leg of Pope, who produced another fine save from the striker's close-range shot on the turn moments later.
Southampton battled throughout but Newcastle's defence is so tight that you simply have to take chances when they present themselves. They could not do it and will now travel to Tyneside as underdogs in the battle for a place in the final.
Player of the match
PopeNick Pope
Southampton
Squad number8Player nameWard-Prowse
Squad number2Player nameWalker-Peters
Squad number31Player nameBazunu
Squad number9Player nameA Armstrong
Squad number22Player nameSalisu
Squad number26Player nameAlcaraz
Squad number18Player nameMara
Squad number27Player nameDiallo
Squad number10Player nameAdams
Squad number23Player nameEdozie
Squad number4Player nameLyanco
Squad number11Player nameOrsic
Squad number19Player nameDjenepo
Squad number45Player nameLavia
Squad number6Player nameCaleta-Car
Squad number35Player nameBednarek
Newcastle United
Squad number22Player namePope
Squad number14Player nameIsak
Squad number7Player nameJoelinton
Squad number2Player nameTrippier
Squad number39Player nameBruno Guimarães
Squad number10Player nameSaint-Maximin
Squad number4Player nameBotman
Squad number5Player nameSchär
Squad number24Player nameAlmirón
Squad number33Player nameBurn
Squad number23Player nameMurphy
Squad number36Player nameS Longstaff
Squad number28Player nameWillock
Squad number9Player nameWilson
Line-ups
Southampton
Formation 4-2-3-1
31Bazunu
2Walker-Peters4Lyanco6Caleta-Car22Salisu
26Alcaraz27Diallo
19Djenepo8Ward-Prowse11Orsic
18Mara
31Bazunu2Walker-PetersBooked at 44minsSubstituted forBednarekat 88'minutes4LyancoBooked at 84mins6Caleta-CarBooked at 86mins22SalisuBooked at 90mins26AlcarazBooked at 37mins27DialloSubstituted forLaviaat 82'minutes19DjenepoSubstituted forA Armstrongat 45+4'minutes8Ward-Prowse11OrsicSubstituted forEdozieat 57'minutes18MaraSubstituted forAdamsat 57'minutesSubstitutes
3Maitland-Niles9A Armstrong10Adams13Caballero15Perraud23Edozie24Elyounoussi35Bednarek45LaviaNewcastle
Formation 4-3-3
22Pope
2Trippier5Schär4Botman33Burn
36S Longstaff39Bruno Guimarães28Willock
24Almirón9Wilson7Joelinton
22Pope2TrippierBooked at 70mins5Schär4Botman33Burn36S Longstaff39Bruno GuimarãesBooked at 86mins28WillockSubstituted forSaint-Maximinat 67'minutesBooked at 84mins24AlmirónSubstituted forMurphyat 78'minutes9WilsonSubstituted forIsakat 67'minutes7JoelintonSubstitutes
6Lascelles10Saint-Maximin11Ritchie12Lewis14Isak19Manquillo23Murphy26Darlow32AndersonLive Text
Match ends, Southampton 0, Newcastle United 1.
Second Half ends, Southampton 0, Newcastle United 1.
Mohammed Salisu (Southampton) is shown the yellow card for a bad foul.
Alexander Isak (Newcastle United) wins a free kick on the right wing.
Foul by Mohammed Salisu (Southampton).
Attempt missed. Alexander Isak (Newcastle United) right footed shot from a difficult angle on the right misses to the right. Assisted by Bruno Guimarães.
Attempt blocked. Allan Saint-Maximin (Newcastle United) right footed shot from more than 35 yards is blocked. Assisted by Nick Pope.
Attempt missed. Jacob Murphy (Newcastle United) right footed shot from the right side of the box is close, but misses to the right. Assisted by Bruno Guimarães.
Sven Botman (Newcastle United) wins a free kick in the defensive half.
Foul by Che Adams (Southampton).
Attempt missed. Kieran Trippier (Newcastle United) right footed shot from outside the box is just a bit too high from a direct free kick.
Substitution, Southampton. Jan Bednarek replaces Kyle Walker-Peters.
Bruno Guimarães (Newcastle United) is shown the yellow card.
Second yellow card to Duje Caleta-Car (Southampton) for a bad foul.
Allan Saint-Maximin (Newcastle United) wins a free kick in the attacking half.
Foul by Duje Caleta-Car (Southampton).
Allan Saint-Maximin (Newcastle United) is shown the yellow card.
Lyanco (Southampton) is shown the yellow card.
Bruno Guimarães (Newcastle United) wins a free kick in the attacking half.
Foul by Lyanco (Southampton).