The Arsenal defenders are like two peas in a pod when they line up alongside one another for Arsene Wenger's side, forming a formidable partnership that hast kept club captain Thomas Vermaelen largely out of the team.
This week, however, they will finally face one another as France take on Germany in the World Cup quarter-finals.
The tie promises to be a mouth-watering one, with France considered a major threat after their tournament so far and Germany having been strongly fancied in the lead up to the Brazilian showpiece event.
But of each side's Arsenal representatives, who has had the better World Cup so far - Mertesacker or Koscielny?
|Team||Won Tackle||Last Man Tackle||Goals Conceded||Clean Sheet||Intercept Won||Effective Clearance||Aerial Duel Won||App||Min|
It's a close call, as Opta stats tell us.
Both centre-backs have had relatively strong tournaments and excelled in key areas. Frenchman Koscielny, who hasn't played every game due to the presence of Liverpool's Mamadou Sakho, has conceded less goals while on the pitch, though both have kept two clean sheets.
Mertesacker, however, is ahead on won tackles - with two - and has made one tackle as the last man. The 29-year-old has also won three more interceptions and made ten more clearances, though that could be attributed to his many more minutes on-field.
Nevertheless, Koscielny has won more aerial duels, despite his reduced playing time - but can also be said to have had to defend less than his German counterpart, with France coming up against less opposition in the group stages.
Koscielny and his team-mates had to produce resolute displays to see off Nigeria in the last-16 but faced little resistance from Ecuador, Honduras and Switzerland beforehand. Mertesacker was kept considerably busier against Ghana and Algeria, with Germany's high line leaving them in all sorts of trouble.
The winner? Mertesacker just, although the stats leave it open to interpretation.
But the duo's similar stats do serve to show just why they work so well together at club level. One attacks the ball and one sweeps, one makes more tackles and one wins more headers. Sadly, though, one will have to leave this World Cup on Friday.