Shinjji Okazaki scored twice in Mainz's 3-1 victory away to Hertha Berlin on Saturday afternoon.
Hertha Berlin have signed former Chelsea forward Salomon Kalou on a three-year contract.
Bayern Munich were held to a 1-1 draw by Schalke, while Bayer Leverkusen ran out 4-2 winners against Hertha Berlin.
Bayer Leverkusen twice came from behind to beat Hertha Berlin 4-2 at the BayArena and maintain their perfect start.
Sky Sports understands Hertha Berlin are leading the chase to sign Lille forward Salomon Kalou.
Hertha Berlin midfielder Alexander Baumjohann has suffered a major setback by tearing his cruciate knee ligaments again.