Mertesacker reckons his partnership with the France ace comes down to team-work.
He said: "We complement each other very well. We are different types of defender, which makes it much harder for the opponent.
"One is maybe more aerial and the other is more mobile, so we have different qualities that work well together.
"But the main part really is to cover each other and to understand each other's game. So we really worked on that - we had to work on that because it wasn't working well from the start.
"But now we are working well together and know each other much better.
"It took two years to achieve that, though, and to feel that we are ready to compete for more than just fourth place."
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