Goals from Vladimir Darida and Salomon Kalou either side of halftime ensured Hertha won their third straight Bundesliga game and their fifth in six outings. They now have 32 points from 17 matches going into the winter break.
The victory put them three points ahead of Borussia Moenchengladbach, who climbed back into the top four with a scintillating 3-2 victory over Darmstadt 98 despite playing for 52 minutes with 10 men following the dismissal of Granit Xhaka.
Mainz, toothless in attack for much of the game, tried belatedly to fight back and hit the crossbar with a 20-metre shot from Pablo de Blasis but could not avoid their first defeat in seven matches.
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Gladbach bounced back from last week's 5-0 demolition by Bayer Leverkusen, snatching victory with Oscar Wendt's 86th minute tap-in following outstanding work from Lars Stindl, to move up to 29 points.
Bayern Munich extended their advantage at the top of the table to eight points with a 1-0 victory at Hanover 96 on Saturday while Borussia Dortmund suffered a last-gasp 2-1 defeat at Cologne.
Leaders Bayern also announced on Sunday that Spaniard Pep Guardiola would be leaving the club at the end of the season, with Italian Carlo Ancelotti coming in to replace him.
(Reporting by Karolos Grohmann, editing by Ian Chadband)
