Manager Arsene Wenger said Arsenal were "in trouble" in their Europa League quarter-final second leg before recovering to beat CSKA Moscow.
The Gunners, 4-1 up after the first leg, were trailing 2-0 in Moscow before Danny Welbeck and Aaron Ramsey scored late goals to seal a 6-3 aggregate win.
"Maybe we were surprised by the intensity at the start," Wenger said.
"They scored the second, had a chance for the third. It was a good response from us after the break."
The hosts, who led through Fedor Chalov and Kirill Nababkin, went close to drawing level on aggregate and leading on away goals as goalkeeper Petr Cech saved Aleksandr Golovin's free-kick and Sergei Ignashevich shot narrowly wide.
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Arsenal reached a European semi-final for the first time since 2008-09, when they lost to Manchester United in the Champions League last four.
"It should be every year - never enough," Wenger told BT Sport.
"You could see in Europe a lot of turnovers. We need to be focused. Let's prepare well for the next round and see who we get."
The Gunners join Atletico Madrid, Marseille and Salzburg in the semi-final draw, which takes place in Nyon, Switzerland, at 11:00 BST on Friday.
Asked if he would prefer to avoid favourites Atletico, who are second in La Liga behind Barcelona, he said: "Ideally, maybe they are the strongest on paper, but let's play well against the team we get."
Arsenal display 'not good enough'
Captain Laurent Koscielny said Arsenal put pressure on themselves by not competing properly in the first half with a spirited CSKA side.
"It was not the plan," said the France defender. "We missed a lot of movement, to be available for our team-mates. We didn't win duels.
"They played well, had opportunities and took them. It put pressure on us - if they score we are out.
"So I am happy because we came back, but our performance was not enough for the stage of the competition.
"We will see the draw, but need more confidence in our game. It is important for the club to go far.
"It is our last chance to qualify for the Champions League, so we will fight until the end."
Welbeck said the Gunners survived a "difficult" night.
"It wasn't comfortable. We have seen in Europe how difficult it can be coming into games with an advantage," he said, referring to a week containing several notable comebacks in the Champions League.
The England striker, who scored twice in Sunday's 3-2 Premier League win over Southampton, added: "It was difficult today - away from home in different conditions - but we stuck through it."