The Black and Yellow's lead at the top was cut to just six points by a shock defeat to Fortuna Duesseldorf in midweek, and second-placed Gladbach will be looking to shorten it even further in the battle of the Borussias - the Borussen derby, this weekend.

That clash presents third-place Bayern Munich with a chance to gain ground on Saturday evening when their coach Niko Kovac also faces a previous employer in high-flying Eintracht Frankfurt.

At the other end of the table, struggling heavyweights Stuttgart and Schalke go head to head, while Werder Bremen are fighting off accusations of drone espionage.

Here are five things to look out for in the Bundesliga this weekend.

- Battle of the Borussias

After suffering their first defeat of the season on Tuesday, Borussia Dortmund are hoping to restore their nine-point lead at the top of the table when they host title rivals Gladbach on Friday.

The first versus second clash will also see Lucien Favre face his former club for the first time since resigning as Gladbach coach back in 2015.

"Gladbach are a very good team, they are compact and they get forward quickly," warned Favre this week.

Both teams are missing key defenders, with Abdou Diallo and Manuel Akanji out for Dortmund, and Nico Elvedi a doubt for Gladbach.

- Kovac returns to Frankfurt

Bayern coach Niko Kovac will also be reunited with his former colleagues this weekend as the champions head to Eintracht Frankfurt for another crucial top-six showdown.

It is the second time that Kovac has returned to Frankfurt since joining Bayern in the summer. The first came back in August, when Bayern romped to a 5-0 win in the German Supercup

Having snatched a late win over RB Leipzig in midweek, Bayern are hoping for a similar result on Saturday evening to keep the heat on Dortmund and Gladbach.

Kovac will have to do without in-form winger Serge Gnabry, who picked up a thigh injury against Leipzig. Kingsley Coman is also a doubt, while Arjen Robben remains sidelined until next year.

- Jovic on fire

While Bayern are running low on attacking options, Frankfurt can boast one of the Bundesliga's most potent goalscorers.

After grabbing a third double of the season against Mainz in midweek, Serbian striker Luka Jovic is now the Bundesliga's joint top scorer with 12 goals in 14 games.

- Schalke in crisis

Schalke head to Stuttgart in a full-blown crisis on Saturday, after being booed by their own fans in midweek.

A 2-1 defeat at home to Leverkusen was Schalke's fourth game in a row without a win, and left them with just 15 points from 16 games, their worst tally at this stage of a season in 25 years.

Defeat to Stuttgart could see last year's runners up slip back into the bottom three, but the club's bosses have insisted coach Domenico Tedesco still has their full backing.

"I find it difficult to question a coach who, just four months ago, we were all celebrating," said sporting director Christian Heidel.

- Bremen's spy drone

Werder Bremen's trip to RB Leipzig on Saturday looks set to be overshadowed by a spying scandal involving a drone.

Bremen were accused of using a flying object to spy on their opponents' training session ahead of their clash with Hoffenheim, and the incident is now the subject of a police investigation and an internal club inquiry.

It is illegal to fly drones directly above groups of people without a permit in Germany, but Bremen's sporting director Frank Baumann shrugged off accusations of foul play.

"Creative methods from analysts are normal, so we are very relaxed," he said.

Fixtures (all times 1430 GMT unless stated)

Friday

Borussia Dortmund v Borussia Moenchengladbach (1930)

Saturday

Bayer Leverkusen v Hertha Berlin, RB Leipzig v Werder Bremen, Stuttgart v Schalke 04, Hanover v Fortuna Duesseldorf, Nuremberg v Freiburg, Eintracht Frankfurt v Bayern Munich (1730)

Sunday

Augsburg v Wolfsburg, Hoffenheim v Mainz (1700)