Taika Waititi's "Thor: Ragnarok" has thundered to one of the year's best box-office debuts in the United States, opening with an estimated US$121 million in North American theaters.

The robust debut for the third "Thor" movie was a welcome shot in the arm for Hollywood and theater owners who have just suffered through a terrible October at the box office.

"Thor: Ragnarok" also bucked the trend of diminishing returns for sequels.

The 2011 "Thor" debuted with $65.7 million; 2013's "Thor: The Dark World" opened with $85.7 million.

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The big opening cements the breakthrough of New Zealand director Waititi, who shepherded the $180 million production to Marvel's best reviews since 2008's "Iron Man."

Chris Hemsworthaas Thor. Photo / AP
Chris Hemsworthaas Thor. Photo / AP

The weekend's other new nationwide release was the holiday-themed comedy sequel "A Bad Mom's Christmas." It grossed $17 million over the weekend.

Waititi has confessed he is an unlikely Hollywood talent.

"I didn't want to do any of this," the 42-year-old has said.

While growing up on the North Island's east coast, "I was doing acting and art, but it was never my dream as a kid. I'm not one of these people who played around with a camera as a kid.

"I fell into this sort of thing," continues Waititi, who calls his career "a mistake."

Just the sort of professional mistake that allows him to proceed with a sense of Zen freedom, he says, because none of this is supposed to be happening.

"Eventually, I made short films that did really well and I was forced to become a filmmaker," says Waititi, whose 2004 live-action short Two Cars, One Night garnered a raft of honours, including an Oscar nomination.

"I forced myself to fall in love with it - it was an arranged marriage."

Waititi delivers those words so evenly that his serious sentiments began to creep toward the deadpan tone that marked his 2014 vampire mockumentary What We Do in the Shadows, co-starring and co-created by friend Jemaine Clement (Flight of the Conchords, Moana), one of his comedy-troupe partners dating back two decades to their days at Victoria University of Wellington.

Waititi followed up that film with last year's Hunt for the Wilderpeople, an adventure comedy starring Sam Neill (who has a Ragnarok cameo) that won a wave of festival awards.

Now, he pivots from films that grossed less than US$7 million to a franchise blockbuster that made that much overseas in just its opening hours.

Yet Waititi says he puts a similar trust in his sense of storytelling, even as executives such as Marvel Studios president Kevin Feige oversee the latest step in the decade-long unfolding of this cinematic universe.

- AP