Mainz's Nicolai Mueller scored his fifth goal in four games but hosts Hanover struck twice late in the first and second halves to climb up to fourth place on nine points, ahead of the visitors on goal difference.
Hamburg SV celebrated their first victory of the season after beating promoted Eintracht Braunschweig 4-0 with two early goals from Rafael van der Vaart and Jacques Zoua.
Hakan Calhanoglou, who came on in the 79th minute, still had time to grab two goals for himself and lift Hamburg to 12th place on four points, easing some pressure off coach Thorsten Fink.
Borussia Moenchengladbach scored five times, including a Havard Nordtveidt own goal to ease past Werder Bremen 4-1 and move up to seventh.
Bayern Munich top the table with 10 points after their 1-1 draw at Freiburg on Tuesday. Undefeated Bayer Leverkusen, on nine points, take on fellow Champions League competitors Schalke 04 later on Saturday.
(Reporting by Karolos Grohmann, editing by Pritha Sarkar)
