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.
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 favorites 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.”