Mertesacker has revealed that losing to Southampton at St Mary's on New Year's Day was the catalyst to their massive improvement over the next few games.
The Gunners have won all their games after the loss, including three consecutive ones in the Premier League and moved into the fifth round of the FA Cup.
Arsenal were helped by the return of their best players, while in the Saints game, they were forced to play Calum Chambers in midfield owing to injuries sustained by Mikel Arteta, Mathieu Flamini, Aaron Ramsey, Mesut Ozil and Jack Wilshere.
The Gunners managed to field their first choice back four in the game, including Mertesacker, Mathieu Debuchy, Laurent Koscielny and Kieran Gibbs, but were let down by their goalkeeper Wojciech Szczesny, who made two horrendous errors to hand the initiative to the hosts.
The Pole has since been kept on the bench with David Ospina taking over in the Premier League. The Gunners have also been helped by the return of Ozil, Ramsey and Theo Walcott with Danny Welbeck expected to return against Tottenham Hotspur.
"Southampton was a major setback - it was a bad start to the year. After that, we promised ourselves to come out of that comfort zone and be a bit more focused, especially [to know] that what comes first is defensive work and not just for the back four," Mertesacker told www.arsenal.com.
"That was maybe the main thing for us to focus on, and afterwards we played more convincingly, especially defensively. You can see the benefit when everyone realises that, and I thought everyone realised it very quickly after we lost at Southampton.
"Maybe sometimes you need a wake-up call to realise what is really important. We worked a lot on our defensive shape, not just the back four, to avoid those dangerous free balls against us. We look much better and the best game defensively was against Man City - that was a bit of a change.
"In the last few years, we went there and wanted to convince everybody that we could play our own way, so it was a bit of a change [this time]. Sometimes we have to do something unexpected, and that was maybe the winning moment," he added.