Hertha Berlin host capital-city rivals Union Berlin behind closed doors at the Olympic Stadium in Friday's derby, with former Germany striker Max Kruse in stunning form for the visitors, who are on an eight-match unbeaten run.

This will be the third Bundesliga clash between the clubs since Union's promotion in 2018/19 and the second behind closed doors due to the coronavirus pandemic.

East Berlin-based Union won a fiery confrontation at home last season with a 90th-minute penalty in November 2019, only for Hertha, who play in the west, to exact revenge with a 4-0 thumping in May.

To make their presence felt, Hertha had tens of thousands of flags in the club's blue and white colours planted across Berlin on Monday.

"We're playing a derby without fans, we wanted to create a little derby atmosphere, a little derby fever," a Hertha spokesman said on Wednesday.

Rival fans were soon posting pictures either posing proudly with their flag or dumping them in the nearest bin -- depending on their loyalties.

However, the PR stunt happened without the necessary permit, putting Hertha offside with city authorities, who threatened to fine the club.

Union's unbeaten run has taken them to sixth in the table, six points behind leaders Bayern Munich, who they host the following weekend.

In contrast, Hertha are 13th and yet to win a home game this season, having been thrashed 5-2 by Borussia Dortmund a fortnight ago with Erling Braut Haaland scoring four goals.

Union took a gamble this season by signing the ageing Kruse, 32, a renowned card player who has competed in the World Series of poker, but the transfer has paid dividends.

He made the last of his 14 appearances for Germany five years ago, yet Kruse has banged in five goals and made four assists in his last four games.

He came to the rescue last Saturday by scoring twice, including a late equaliser to salvage a point in a 3-3 home draw with Eintracht Frankfurt.

"Hopefully Max will be important in the derby," said Union coach Urs Fischer.

"He's a great guy and he's shown that he knows when he has to work hard. I'm happy. He's there for the team and it's gone exactly as I expected."

Fischer's opposite number Bruno Labbadia made it clear how difficult it will be to stop the Union forward.

"You have to keep him in check collectively," added Labbadia, who is without injured Argentine midfielder Santiago Ascacibar and Colombia striker Jhon Cordoba.