Arsenal manager Arsene Wenger has insisted nothing is decided in Group F as the Gunners' 1-0 Champions League win at Borussia Dortmund kept them top only on goal difference.
Welsh midfielder Aaron Ramsey headed the crucial second-half goal as Arsenal avenged their 2-1 defeat at home to the Germans a fortnight ago.
Arsenal became the first team since Stuttgart in November 2012 to prevent Dortmund scoring at home, but stay top of Group F on goal difference with nine points and level with Napoli.
"It's difficult to say how the group will go now," insisted a cautious Wenger, whose side host Marseille in a fortnight, then travel to Napoli in December.
"We have to win our home game, just like Dortmund, there is still a lot of football left to play.
"It was a difficult win for us, but we took our chances."
Dortmund had 13 shots compared to Arsenal's four, but the visitors made their chances count.
"It was a tough game, Dortmund had better chances in the first half, but we took ours and we weren't under so much pressure once we went ahead," added Wenger.
"Dortmund fought fantastically hard and had us under pressure, but we managed to get the goal.
"We were mature tactically, didn't make many big mistakes and we showed patience.
"We had a tough phase at the start of the second half and after we went 1-0 up we weren't under as much pressure and even had our chances for a second goal."
Having dropped to third in the table, last season's Champions League finalists Dortmund possibly having to win both their remaining games at home to Napoli, then away to Marseille to keep alive their hopes of reaching the knock-out stages.
"Losing the game was unnecessary, but we haven't lost hope yet, everything is still possible as long as we win the remaining two games," said Dortmund coach Jurgen Klopp.
"It's definitely going to get exciting from now on.
"I told the team, what everyone saw today, that we weren't rewarded for our efforts.
"We hadn't planned on losing the game and it wasn't necessary.
"But it's not over, we still have every chance of two wins."
Captain Roman Weidenfeller was furious with the result of the match played in heavy rain.
"Football can be unfair at times," said the goalkeeper.
"We had a bit of luck in London and we were punished for it tonight.
"Unfortunately, we lacked a bit of purpose going forward and we wanted to do things a bit too nicely.
"In weather like this, you have to shoot a bit more. The group is wide open now."
Arsenal's Germany centre-back Per Mertesacker, who kept Dortmund's Poland hot-shot striker Robert Lewandowski subdued, said the Gunners had simply stuck to their pre-match plan to contain Borussia's potent attack.
"We wanted to restrict the way Dortmund ran at us," said the towering defender.
"We did that well in the first half and in the second half we used our chances to counter-attack.
"We had a bit of luck, but things worked out well for u."