As the Denver Broncos prepared to battle the New York Giants on Sunday Night Football, China's top NFL announcers were going through their own pre-game rituals.

For Wang Zixing and Xiao Shen, game day meant waking before dawn and driving to the Beijing studio where they host a four-hour live streamed broadcast of NFL games for Chinese commuters – one of the fastest-growing fanbases of American football.

"From September to June, you have to get used to getting up very early," said Mr. Wang, now beginning his third year as an NFL commentator.

American football is less popular than basketball and soccer in China, but fan numbers are growing quickly, as young people accustomed to streaming sports on mobile telephones get drawn in.

The NFL has been quick to seize on the trend, with February's Super Bowl championship game viewed by roughly 1.5 million online viewers in China.

This season the NFL scrapped a mix of online streaming deals in favor of an exclusive digital partnership with China's top social media and gaming firm, Tencent Holdings Ltd.

But the sport's rapid growth and youthful fan base have exposed a shortage of Mandarin-speaking commentators who have the football knowledge to call a game.

Wang, who started as a commentator on National Basketball Association (NBA) games, moved into American football by accident when producers asked him to fill in at the last minute.

"I was very nervous and worried that I might explain the rules wrong," he said of that first experience.

Now, he says, translating the jargon is the toughest thing about explaining the game to China's young football fans.

For example, Wang said, he would sometimes use a basketball term, "pick and roll", to explain "blocking" in American football.

Ahead of Sunday's broadcast, Wang and Xiao spent time in the makeup room before running through the day's other NFL scores with the director.

They work at an anchor desk festooned with NFL logos, video screens, and props, including a football and helmets of the two teams.

Broadening its online presence isn't the only play the NFL is dialing up in China.

In June, five-time Super Bowl winner Tom Brady caused a stir on Chinese social media when he tossed a football on the Great Wall in a goodwill visit to promote the sport.

But even a visit from the New England Patriots quarterback couldn't punt the game's popularity above basketball. The NBA final, also broadcast on Tencent's platform, was watched by a record 65.9 billion Chinese viewers in 2016.

"I'm a big fan, but if I'm honest I don't always get the rules," said Wang Ming, a technology blogger who began watching the NFL live stream this year.

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"Maybe that's why I like it. It's just totally weird to me."

This story was reported by Reuters.