Arsene Wenger's side have been in superb form this season and top the table with 35 points gained from 15 matches.
But the Emirates Stadium outfit will face a stern test of their title credentials in December, with Saturday's visit to Manchester City followed by a home fixture against Chelsea.
Wenger's charges have lost just twice in the top flight this term, and centre-back Mertesacker feels they are in good stead after overcoming their "toughest game" of the campaign in a 1-1 draw with Everton on Sunday.
"I think that was the toughest game we have been through the whole season," the Germany international told Arsenal's official website. "They pressured us high up, all the way through from the start until the end.
"We couldn't play that much and in the second half everything was possible on both sides, with chances here and there so I think it was a fair result.
"We have tough games coming up over the next week and over the Christmas period. We just want to go one game at a time.
"We are really relaxed, we don't have to respond to anyone. We just have to do our job every single second of the game. That is our target, nothing else."
But Roberto Martinez's side secured a share of the spoils thanks to substitute Gerard Deulofeu's strike four minutes later.
