Thomas Tuchel's side are now just five points behind the Bundesliga leaders after their victory with the two sides set to clash at Signal Iduna Park on Saturday evening

Borussia Dortmund gained vital ground on Bayern Munich ahead of Saturday's Der Klassiker with a 2-0 win over Darmstadt on a night when their arch-rivals slipped up.

While Bundesliga leaders Bayern were stunned by a 2-1 home defeat to Mainz, Thomas Tuchel's side dominated struggling Darmstadt and scored either side of half-time to claim a victory that moves them to within five points of the reigning champions.

And that gap could be down to just two points at the weekend, with Bayern visiting Signal Iduna Park for a mouthwatering clash that looks set to go a long way to deciding the destination of the title.

A win for Dortmund at Merck-Stadion am Bollenfalltor rarely looked in doubt, but the visitors were made to work hard before Adrian Ramos opened the scoring after 38 minutes.

Erik Durm added a second by finishing off a fine team move eight minutes after the restart, with Tuchel's decision to rest the likes of Henrikh Mkhitaryan and Shinji Kagawa fully vindicated by a commanding performance that made it six wins on the bounce in all competitions.

Dortmund, making their first visit to Darmstadt in 34 years, tested goalkeeper Christian Mathenia with a curling Ilkay Gundogan free-kick after five minutes, but the embattled hosts started brightly at a packed Merck-Stadion am Bollenfalltor.

Tuchel handed a full debut to teenage winger Felix Passlack, who saw plenty of the ball as Dortmund began to dominate possession.

Darmstadt succeeded in frustrating Dortmund for more than half an hour and striker Sandro Wagner came within inches of scoring when he controlled the ball perfectly with his chest before unleashing a low drive that flashed just wide of the post with Roman Weidenfeller beaten.

But Dortmund edged in front moments later when Gonzalo Castro's cross was met by Pierre-Emerick Aubameyang and the rebound from his saved header was prodded home by Ramos with Mathenia prone on the turf.

Aubameyang should have doubled Dortmund's lead just after half-time when he had a sliding attempt at goal blocked on the line, but moments later Castro's through ball found Durm racing into the box and the left-back took a touch before side-footing it into the net.

Darmstadt refused to concede defeat, and they came close to pulling a goal back when Wagner headed the ball down to Mario Vrancic in the box, but the striker blazed over the crossbar under pressure from Mats Hummels.

Aubameyang should have ended the game with a goal when he broke clear and went one-on-one with Mathenia but the goalkeeper did just enough to block his shot and keep the final score at 2-0 on an evening when the Bundesliga title race was blown wide open.