Mertesacker began his senior career after being promoted from the youth ranks at Bundesliga club Hannover 96, making his senior league debut in November 2003. He was soon dubbed "the Defence Pole" ("die Abwehrlatte") by German tabloids, and gained a reputation for his exceptional disciplinary record, going 31 league games without being booked. After various standout performances at Hannover, as well as starring for the Germany national team at the 2006 FIFA World Cup at age 21, he transferred after 74 league matches for "96" to fellow Bundesliga outfit Werder Bremen, for a fee of €5 million later that summer. His tenure at the club was characterized for his ability to read play and stifle the build-up play to opposition attacks, establishing himself as a defender with a quiet-but-effective game, and a core-component of the Bremen team that won the 2009 DFB-Pokal, and finished as runner-up in the 2009 UEFA Cup.