Japanese video game giant Nintendo is banking on the latest instalment of its 40-year-old saga "Zelda" to breathe life into its business, after forecasting a drop in profits this year.

The firm will release "Tears of the Kingdom" on Friday, with clips circulating on the internet already racking up millions of views and generating feverish excitement among fans.

The saga of Princess Zelda and the elf-like warrior Link has shifted 125 million copies worldwide since its first edition in 1986.

It helped to forge "open world" games where the player is free to roam in virtual landscapes -- an idea later taken up by games from "Grand Theft Auto" to "Skyrim".

But its main challenge this year will be to boost the figures for the Japanese studio and prolong the life of its Switch console, which experts say is in its dotage after seven years on the shelves.

The game "will be by far the biggest contributor to Nintendo's sales this year", said Serkan Toto, an analyst at Kantan Games.

Yet the franchise was something of a risk during the 1980s for a company that also boasts the "Super Mario" series as part of its stable.

- 'Pioneer' -

The first episode, "The Legend of Zelda", plunged gamers into an unknown universe largely without instructions.

Creator Shigeru Miyamoto, who also gave life to Mario, was inspired by his childhood explorations of the Japanese countryside to offer a landscape of forests, lakes, caves and mountains.

"The scale of the game was huge at a time when most games were finished in an hour or two," said Kiyoshi Tane, an author specialising in the history of video games.

"The map was designed with a real emphasis on exploration, so it was something of a pioneer of what open-world games would become."

The first Zelda hit the market only a few months after "Super Mario Bros", but the two games were far apart on the gaming spectrum.

While Mario runs from left to right through various platforms, Zelda "encouraged the player to explore, discover and map its world and take on its challenges", said Mark Brown, who analyses game design on his YouTube channel.

The game was a smash hit from the off, and for the next two decades it pushed the boundaries of game design.

The 1998 edition "Ocarina of Time" pioneered a system to allow gamers to aim properly in 3D.

Yet the game eventually hit the skids at the turn of the 2010s.

- 'High bar' -

Nintendo wanted to expand the game's appeal but only managed to create editions that satisfied nobody.

Hardcore fans drifted away and sales suffered. 

"The development team had a sense of crisis," Katsuhiko Hayashi of trade magazine Famitsu told AFP.

The designers rethought the basics of the game, eventually creating 2017's "Breath of the Wild".

It was launched alongside the Switch console and has since become the best-selling edition of Zelda.

"This game set a high bar for the open-world action-adventure genre, and Zelda is still at the top," said Hayashi.

Zelda has become something of a "bible" for other developers, he added.

But despite this success and the enduring popularity of Nintendo's other franchises -- demonstrated by this year's smash hit "Super Mario Bros. Movie" -- the firm released a gloomy outlook on Tuesday.

The company forecast a drop in net profit by 21 percent this year compared with last year.

The prediction makes the Zelda release vital for the company.

Charles-Louis Planade, an analyst at Midcap Partners, reckons it could become "the best-selling game in history". 

"It's a game that can approach $1 billion in revenue, which is very significant for a company that makes a turnover of just over $10 billion per year," he told AFP.

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