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Dortmund, which will host Athletic Club in the UEFA Champions League this week, was without top striker Serhou Guirassy, who was injured during the warm-up.

But it still got the result away from home to go 13 league matches without a defeat, going back to last season.

Dortmund has not conceded a league goal since the first matchday on 24 August (AEST).

Nico Kovac's side led at half-time for the 10th Bundesliga match in a row, setting a new league record.

Victory also maintained the pressure on league leader Bayern Munich after its 4-0 win over Werder Bremen, with Dortmund now on 13 points in second place, two behind the defending champion.

"It happened exactly the way we had wanted it to," Kovac said after the win.

"The team played in a very disciplined and organised way. We scored at the right times and today I saw the team play out an away game as we should.

"We are only looking at us. I know that football in Germany has expectations that Bayern do not march away on their own this season. But we have to keep things in perspective. It is the fifth matchday and still early in the season. But we are doing it well so far."

The visitors dominated from the start and went ahead in the 27th minute when Svensson at the far post tapped in a Julian Brandt cutback.

The goal came seconds after the hosts had hit the woodwork through Paul Nebel.

Germany international Brandt then delivered another assist after Adeyemi sprinted clear and finished the job in the 44th.

Mainz was left with 10 players when keeper Robin Zentner was sent off after bringing down Adeyemi outside the box but by that time the game had long been decided with Dortmund in complete control.

Brandt nearly scored himself but his stoppage-time effort was spectacularly palmed over the bar by substitute keeper Lasse Riess.