The Gunners recorded a comfortable 3-0 victory at home to Dinamo Zagreb on Tuesday night meaning they now head to face Olympiacos in Athens in a fortnight with progression to the knock-out rounds in their own hands.
That was not the case heading into a nervy contest against the Croatian champions as Arsene Wenger's side had lost three of their four previous Group F matches.
Needing Bayern Munich to beat Olympiacos in Germany and keep up their end of the bargain at the Emirates, Arsenal got what they needed as Bayern cantered to a 4-0 success to top the group while a Mesut Ozil header and two goals for Alexis Sanchez sealed a vital three points.
Having lost in Zagreb and at home to Olympiacos, as well as being thrashed 5-1 in Munich, Arsenal's start to the campaign left them in a position that still requires them to win by two goals in Athens - but Mertesacker believes the squad now feel they can go on and get the required result.
"We believe and before the game we knew we were capable of doing special things," he said.
"We dropped a lot of points which is disappointing but there is still a chance so we believe
It is a nightmare how we played the first two games.
"We have to face the truth and that is that we have to score more than one goal against Olympiacos."
German World Cup winner Mertesacker also feels Olympiacos will be under more pressure as they can afford to lose by the odd goal - as long as Arsenal score less than three - and progress.
"We have a good chance," he added.
"We believe that we can score goals
You saw that we are always capable of doing that
Let's go there without fear, respect our opponent but keep patient until the end."
Ozil and Sanchez, Arsenal's two biggest signings in their history, showed their class to steer Arsenal to the win as both excelled in their own position.
Before he headed in the opening goal Ozil had been instrumental and shown off an array of passing that has seen him rack up assist after assist this season.
Sanchez meanwhile was full of life, quite the accolade for a man who could easily be battle-worn after an extended spell in the sides of both club and country, and his willingness to run, hassle and battle for possession does not go unnoticed.
"They got on with each other and we are happy for them," Mertesacker said of the influential pair.
"They could have scored even more but obviously we need them in good shape and with a good understanding for the rest of the season.
"We needed to win
Everyone was on the ball from the start, we attacked well and that is something we need to continue if we want to compete for anything this season."
Arsene Wenger has overseen successful progression from Champions League groups for the last 16 seasons but Mertesacker does not want history to weigh down the players ahead of their task in Athens on December 9.
"There is enough pressure
We do not need to discuss that too much," he said.
"They have more to lose
We are in a more difficult situation
It will be pressure for both teams and hopefully we can handle that better."
Source : PA
Source: PA