Mainz missed the chance to go top of the table after losing their first game of the season at Borussia Monchengladbach.
Raul Bobadilla put the home side in front after 29 minutes and Marco Reus eventually sealed all three points with a second six minutes from the end.
Monchengladbach moved into a four-way tie at the top of the Bundesliga table on seven points, but the other three teams are yet to play this weekend.
Mainz came into the game on a high after their 2-1 win over Bayern Munich, but had already seen Bobadilla go close from a free-kick before he opened the scoring.
Bobadilla controlled a long ball into the penalty area and rounded the goalkeeper before scrambling the ball into the net from six yards.
The visitors could have got back on level terms before the break but Florian Heller fired wide from close range before Heller's cross was acrobatically flicked wide by Tim Hoogland.
Gladbach almost doubled their lead after 65 minutes when Juan Arango struck a left-footed free-kick off the post.
Bobadilla then saw his shot cleared off the line by Peter Van Der Heyden before Reus made sure of the win with a superb individual effort.
Reus made a great run down the centre of the pitch before firing home from 12 yards to make it 2-0 and send the vast majority of a crowd of 42,217 home happy.