When Fortuna Dusseldorf host Hamburg in Germany’s second tier on Sunday, there will be 52,000 spectators inside the stadium. They could have filled it more than twice over. There were 135,000 ticket applications. Because entry for this game is free.

After trialling the concept for three games last season, with a total of 350,000 applications for those games, Fortuna have extended it to four this time around. And it could soon be coming to the Bundesliga. Seven games in, Fortuna are top of the table.

Alexander Jobst, the club's chief executive officer, is reluctant to call it a success just yet, but his enthusiasm gives the game away. Supporters have largely embraced the concept and the desired growth has been achieved. Average attendances are up to 40,000.

Importantly, overall revenue has increased despite - or should that be because of - the free-ticket offer. Fortuna are finding new fans, the whole relationship between club and city transformed. There are now 35,000 members and more joining every day.

"We did something new, introducing a concept which nobody had ever tried before," Jobst tells Sky Sports. "It is a transformation of normal football business in Germany. We opened football up for everyone and now people believe in the club again."

Jobst talks with passion about how this is about more than just free games. Projects are ongoing, an emphasis on social impact, with investment in the women's team and the academy. But this is no moral crusade. Necessity was the mother of this reinvention.

"Business as usual was not an option for us. With the old business model, without the investors, just with normal sponsorship and an average attendance of 25,000 supporters at the time, we realised that we could not continue like this."

Fortuna was not exactly a model club. A traditional powerhouse, having won the German cup back-to-back in 1979 and 1980, they narrowly avoided bankruptcy at the start of this century. They do not own their own stadium and the rents are still rising.

It was a predicament for a club squeezed by more successful neighbours and with aspirations for so much more. Dusseldorf is a big city, a prosperous city, and Fortuna is the club of that city. It craves a top-flight side but had no idea how to make it happen.

"When I started at Fortuna, many people were telling me this club is a sleeping giant. Honestly, I cannot listen to this expression anymore. Fortuna Dusseldorf is not a sleeping giant. If it were a sleeping giant, it would have woken up 20 years ago."

A radical idea was required, something that would spark an upturn in interest and set the club on a new trajectory. It does not get much more radical than letting people in for free - and convincing the sponsors to pay for it. But Fortuna have managed to pull it off.

"We had to admit that our stadium, with a capacity of 52,000 spectators for a second-division football club in Dusseldorf, it was a little bit too big. This was a good way to enlarge our fan base, attract new fans to Fortuna Dusseldorf for the future."

Inevitably, there was criticism from outside. Some were concerned that the concept was rooted in commercialisation, others that it raised issues around fairness. And those concerns were felt within the club too, as Jobst is now prepared to acknowledge.

"Obviously, in any change, any transformation that you are starting, you have criticism, you have many, many questions. Also, we didn't have all the answers." Concerns were varied. One was how this would be received by those supporters already willing to pay.

That worry has gone, only a fraction of the 20,000 supporters recently polled on the matter expressing any negative sentiment. "That's important. We belong to our fans and members." The vast majority are embracing the thrill of being part of bigger crowds.

"There is nothing better for players and supporters than an exciting 90 minutes in a full stadium." Most are enjoying the warm glow of widespread praise too. "We allow people who cannot afford it to feel the atmosphere. That concept itself attracts people."

Sponsors, too. This is a feel-good story, which helps to explain why so many want to be associated with it. The venture is being funded by organisations ranging from Targobank, and Hewlett Packard Enterprise to the social movement Common Goal.

Another thing that had kept Jobst up at night ahead of that first game was the thought that some beneficiaries of the free tickets would not bother showing up. "There's an expression in Germany which says, what does not cost anything is not worth anything."

In fact, the no-show rates for the free games are lower than for the matches for which supporters have paid. Do the new fans return to the stadium having had a taste of it? That question too has been answered. "They are coming back, even if they have to pay."

Now, Jobst describes seeing the smiles on faces at that first free game against Kaiserslautern in October of last season as the best moment of his career with the club. Although, that was not how he was feeling when Fortuna fell three goals behind.

"After 35 minutes, I anticipated that I would have a different job a few days later. All the magazines and all the newspapers would have had the headline already prepared, saying that this was a big failure." Remarkably, Fortuna turned it around to win the game 4-3.

Of course, there is the helpful fact that Fortuna keep winning. They narrowly missed out on promotion last season following a dramatic collapse in the play-off. "A day we don't talk about," says Jobst. But that has not curtailed the momentum. It is a club on the up.

"Sporting success is always a driver of new support," says Jobst. "Merchandising is growing, membership is growing, sponsorship revenues are growing. We would like to convince more to come on board so that we can allow more free matches in the future."

If they can find the sponsors, there is even talk of rolling it out for every home game by 2028. "We don't believe this concept could work everywhere." But it works for Fortuna. Through an imaginative pivot, they find themselves on course to reach the Bundesliga.

When they do, they will do so as one of the best supported clubs in the world's best supported league. A status achieved as a result of the radical thinking that has revolutionised this football club. Given his views on sleeping giants, Jobst is aware of the irony.

"With this concept, we awoke something."

Watch Fortuna Dusseldorf vs Hamburg live on Sky Sports Football YouTube and the Sky Sports App this Sunday; kick-off 12.30pm