Viktor Gyokores punches the air after scoring against ChelseaGetty Images
At a glance
Ben White, Viktor Gyokeres and Martin Zubimendi all score for Arsenal
Chelsea goalkeeper Robert Sanchez partly at fault for two goals
Substitute Alejandro Garnacho scores twice to keep Blues in tie
PLAYER RATER
Nizaar Kinsella
Football reporter at Stamford Bridge
Goalkeeper Robert Sanchez made errors that contributed to two of Arsenal's three goals in a dramatic Carabao Cup semi-final first leg - but Alejandro Garnacho scored twice to keep Chelsea in the tie.
Sanchez failed to claim Declan Rice's corner, allowing Ben White to head in Arsenal's seventh-minute opener from close range - and the goalkeeper let a cross squirm through his fingers shortly after half-time for Viktor Gyokeres to make it 2-0.
Garnacho pulled one back 12 minutes into the second half, but Martin Zubimendi's brilliant shimmy and finish restored Arsenal's two-goal advantage.
The former Manchester United winger then volleyed in his second of the match after Kepa Arrizabalaga's clearance dropped at his feet.
Chelsea's hopes of reaching Wembley to face either Newcastle or Manchester City remain alive, but they must overturn a one-goal deficit at Arsenal's Emirates Stadium on 3 February.
Aside from Arsenal's opening goal, the first half was a closely contested affair, with Enzo Fernandez and Estevao Willian having efforts saved for Chelsea. Arsenal's Gyokeres, Zubimendi and William Saliba also sent shots either wide or over.
After Chelsea fell 2-0 behind, new head coach Liam Rosenior introduced Garnacho, who pulled one back by guiding Pedro Neto's cross into the net. However, Zubimendi skipped past both Andrey Santos and Wesley Fofana before finishing from just inside the box to restore Arsenal's two-goal cushion.
Garnacho remarkably reduced the deficit again, and Estevao saw an excellent effort from the edge of the box saved, while Gabriel Magalhaes and Mikel Merino both had chances to further extend Arsenal's lead.
Arsenal, who scored their 18th goal from a corner and 24th from set-pieces across all competitions this season, are now unbeaten in 10 matches. Chelsea, meanwhile, have won just three of their past 13 games, with Rosenior losing his first home match, having replaced Enzo Maresca last week.
Chelsea analysis: Rosenior fighting problems beyond his control
Such was the importance of this fixture that Rosenior had been planning for it even before Chelsea's previous match against Championship side Charlton in the FA Cup.
The new head coach wanted to be ready for a major test against the Premier League leaders, who have lost only two games in all competitions this season.
Having focused on mentality and intensity when preparing for an FA Cup tie against second-tier opposition, this was the first genuine tactical examination for the former Strasbourg head coach.
However, none of his injury doubts - Cole Palmer, Reece James or Malo Gusto - were able to feature on the night. In addition, Liam Delap and Jamie Gittens withdrew through illness shortly before kick-off, while Moises Caicedo was suspended.
Despite those absences, Chelsea competed with Arsenal in the first half, but fell behind as Sanchez failed to deal with a cross.
Conceding a second set-piece goal inside Rosenior's first two matches, with Sanchez again at fault for the second, will be frustrating and may prompt questions about the goalkeeping situation.
There were, however, early signs of Rosenior's identity emerging. Fernandez's pressing forced Martin Odegaard into losing the ball in the build-up to Garnacho's first goal, while the winger's second came from a set-piece. Rosenior also deployed Fofana in a dedicated man‑marking role on Odegaard, Arsenal's key creator.
Yet with fans chanting against the club hierarchy and in favour of former owner Roman Abramovich, there remain a host of issues beyond Rosenior's control that could affect Chelsea not just in the EFL Cup, but across the competitions still to come.
Arsenal analysis: A big step for Arsenal even though tie is only at halfway point
Alex Howell
Football reporter
This entertaining victory really highlighted the quality that Mikel Arteta has at his disposal.
The Arsenal manager was able to make eight changes for this game, and bring in his strongest XI, after making 10 changes for Sunday's FA Cup third-round win at Portsmouth.
One of those players who stayed in the side was Ben White, and he started in a back four of Saliba, Magalhaes and Jurrien Timber.
That backline provided an imposing threat from set-pieces and White's opener was the 24th goal Arsenal have scored from a set-piece in all competitions this season, the most by any club in Europe's top five leagues.
It was also a good night for Gyokeres, who scored a goal from open play for the first time in 16 games and provided an assist for Zubimendi's goal and Arteta will hope that it is the confidence boost his forward needs.
The EFL Cup is not Arteta's or Arsenal's main objective this season but getting in a position to win a trophy relatively early would be a huge boost as they try to win the Premier League or Champions League.
Arteta said before the game that an EFL Cup triumph would give his team "a nudge" and "a sense that the objective is really close and that is a massive motivation for everybody".
There's still another leg to go in this tie but this was the first individual semi-final match that Arsenal have won in nine attempts and marks another step in the progression they have made this season.
What's next for both sides?
Chelsea next host Brentford at Stamford Bridge in the Premier League on Saturday (15:00 GMT). On Wednesday (20:00 GMT), Cypriot side Pafos face the west Londoners in the Champions League.
Arsenal travel to the City Ground to face Nottingham Forest in the Premier League on Saturday (17:30 GMT). Arteta's side then face Inter Milan away in the Champions League on Tuesday (20:00 GMT).
Have your say on Arsenal's performance
What did you make of Chelsea's display?
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Player of the match
Martín Zubimendi
7.14
Chelsea
Arsenal
49A. Garnacho
7.01
41Estêvão
6.27
7Pedro Neto
5.83
8E. Fernández
5.49
23T. Chalobah
5.08
20João Pedro
5.01
4T. Adarabioyo
4.98
34J. Acheampong
4.93
17Andrey Santos
4.89
3Marc Cucurella
4.87
21J. Hato
4.79
29W. Fofana
4.73
38Marc Guiu
4.68
62S. Mheuka
4.66
5B. Badiashile
4.45
1Robert Sánchez
3.80
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36Martín Zubimendi
7.14
41D. Rice
7.10
14V. Gyökeres
6.99
4B. White
6.92
2W. Saliba
6.73
6Gabriel Magalhães
6.62
19L. Trossard
6.44
7B. Saka
6.41
13Kepa Arrizabalaga
6.33
12J. Timber
6.32
8M. Ødegaard
6.07
23Mikel Merino
6.05
11Gabriel Martinelli
6.01
29K. Havertz
5.69
9Gabriel Jesus
5.68
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After the opportunity to rate players has closed, the score displayed represents the average from all the submissions by BBC Sport users.
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