Aaron Ramsey made it 6-3 with seconds left to cement a result that was long in doubt.
Arsenal have a 4-1 leading against CSKA Moscow ahead of their second leg in Russian Federation but we've all seen how precarious a three-goal first leg lead can be this week...
Arsenal also reached the semi-finals after recovering from a wobble to draw 2-2 at CSKA Moscow and progress 6-3 on aggregate, while favourites Atletico Madrid reached the last four after losing 1-0 at Sporting to go through 2-1 over the two legs.
"You could see when you have a big difference after the first game it is hard to turn up with the same urgency than if the difference is very small and that played a part in the first half", Wenger said.
But with the likes of Atletico Madrid and Olympique de Marseille in Friday's draw, Koscielny said Arsenal can ill afford to take their foot off the gas as they appeared to do in a lacklustre opening hour in Moscow. Arsène Wenger's team are the competition's most prolific side, notching 27 goals already in eight outright wins in eight games.
The Arsenal skipper criticised his side's performance as they scraped past CSKA Moscow
"You could see in Europe a lot of turnovers".
We are in a position where I don't think we are under threat of complacency; it is an important target for us to go as far as possible.
"That is something that is really exceptional that you do not find in many players".
"We didn't win duels".
"No, not at all", he replied when asked by Press Association Sport if the political climate had put him off making the trip.
"This is obviously the only route into the Champions League, so it is the be all and end all and it is a chance of winning a trophy which, as a football fan, is what you want to see".
Welbeck said the Gunners survived a "difficult" night.
Trump reportedly plans to pardon Scooter Libby
The White House stated there there have been no talks of pardoning Libby "at this time". He was sentenced to 30 months in prison in the case, but President George W.