Newcastle United manager Eddie Howe said the tie against Arsenal was "finely balanced" despite taking a giant step towards the League Cup final with a 2-0 win on Tuesday.
Goals by Alexander Isak – his 50th for the club – and Anthony Gordon either side of half time in North London gave Newcastle a seventh successive win in all competitions and put them in the driving seat for the second leg at St James' Park in February.
On all eight occasions since 1987/88 that an away team held a two-goal lead from the first leg, that team went on to reach the final and Arsenal have a mountain to climb if they are to keep alive their hopes of a third League Cup triumph.
Howe also took Newcastle to the League Cup final in 2023 but they fell short of a first trophy since 1955 as they were beaten by Manchester United at Wembley.
Newcastle fans celebrated at the final whistle, but Howe urged caution, saying there was still plenty of work left to set up a final against Liverpool or Tottenham Hotspur.
"Today is another big step forward when you look at the recent run we've had away from home," Howe told Sky Sports. "We played with real confidence and belief but in terms of the tie it's still very much alive and finely balanced and we have a really tough home leg to come."
Howe admitted his side were creaking in the second half as Arsenal desperately sought a way back, but said he was proud of the way his team defended.
"There was a lot of fatigue in the second half," he said. "We started off on the front foot but then we fatigued and then it was about our mentality to defend. We knew what was at stake and the players put their bodies on the line."
Arsenal were left to rue their failure to convert several good chances with Gabriel Martinelli denied by the woodwork in the first half and Kai Havertz missing a golden opportunity.
Arsenal had been unbeaten in their previous 13 games in all competitions but were not at their best against a clinical Newcastle side who have taken a liking to North London.
Three days ago they won 2-1 at Tottenham to move closer to the Premier League's top four and they returned to send the travelling Toon Army back up north in high spirits.
Isak has been virtually unstoppable of late and he has now scored 10 goals in his last nine appearances for the club.
The Swede lofted one early half-chance over the crossbar but showed how lethal he is after 37 minutes when a long ball into the area fell into his path and he fired the ball high past David Raya into the roof of the net.
It brought up the 25-year-old's half century in just 89 matches for the club, five games fewer than it took Newcastle great Alan Shearer.
"First half he was electric, I thought he played really well. He scored but I just thought his general game was in a really good place," Newcastle manager Eddie Howe said.
Arsenal had threatened with Martinelli curling a shot against the post from the edge of the area and Newcastle were forced into some desperate defending in one goalmouth scramble.
Jurrien Timber also headed over the crossbar from close range for the hosts and Havertz was even more culpable after the break when he somehow failed to connect with a header with the goal gaping, the ball bouncing off his shoulder.
By that time, Newcastle were 2-0 ahead thanks to a tap-in by Gordon in the 50th minute. Jacob Murphy threaded a great pass to Isak in the penalty area and his first-time shot was only parried by Raya for Gordon to sweep in the rebound.
Arsenal, who had 23 shots to Newcastle's seven, piled on the pressure late on to try to reduce the deficit but Newcastle manned the barricades to take a healthy lead to St James' Park.
"In every other aspect we were the better team but it's about scoring goals," Arsenal manager Mikel Arteta said. "Credit to them, they defended their box very well. We had the chances but at this stage, a semi-final, you have to be clinical."
