Arsene Wenger admitted Arsenal were "on the ropes" against CSKA Moscow before a late salvo secured a 2-2 draw in Thursday's second leg and a 6-3 aggregate win in the Europa League quarter-final.
The Gunners took a 4-1 first-leg lead to Russia but found themselves 2-0 down and on the back foot early in the second half, with Fedor Chalov and Kirill Nababkin taking CSKA to within one goal of going through.
Wenger had switched to a five-man defence at the break but his side were consistently under pressure, until Danny Welbeck struck a fine goal 15 minutes from time to kill the home side's momentum.
Aaron Ramsey levelled the second-leg scoreline in injury time to spare the Gunners' blushes further, but Wenger accepted the tie could have run away from his side.
April 12, 2018
"Maybe we were a bit surprised by the intensity of the game and were not at the races, in the first half especially," he told 'BT Sport'.
"They scored the second and Ignashevich had a chance to score goal three, then we were a bit on the ropes. But we had a good response: we were better organised in the second half than in the first, our distances were too big.
"If you look at the game, we were in trouble in the first half. Once we moved to a five [at the back], we were more stable and didn't give so many chances away. The second goal comes from a long-distance shot but, in the box, we didn't give anything away any more.
"We have players who can always score. Once we scored the 2-1, it was a big blow for them, and they didn't recover from that."
Arsenal join Marseille, Atletico Madrid and RB Salzburg in the semi-finals.