"Knocking Avatar off its perch atop the box office mountain has been attempted by many films. No movie as of yet has been able to reach the heights attained by that box office juggernaut almost a decade ago," say Paul Dergarabedian of Comscore.

"However, lest anyone think this is an easy road even that Endgame defied box office gravity, pushing past $2 billion is like running the three-minute mile or breaking the sound barrier. Getting in the extra $800 million over the $2 billion it's guaranteed to make will take a superhero-like effort considering the competition on the horizon," Dergarabedian says.

He and other analysts, however, are far comfortable predicting that Endgame could pass up Titanic to become the No. 2 film globally with $2.3 billion to $2.5 million.

Avatar, Titanic ($2.19 billion) and Star Wars: The Force Awakens ($2.07 billion) all opened in December, and were able to hang onto theaters well into the new year. In late April 2018, Avengers: Infinity War — which debuted to a then-record $257.7 million in North America and $640 million globally — topped out at $2.06 billion.

Like Infinity War did, Avengers: Endgame kicks off the summer box office, a far more crowded frame.

To match or best Avatar, Endgame would need to do more than 2.3 times its opening, a reasonable expectation for many successful tentpoles. However, no film has ever opened to this level.

No one, including Disney executives, will have a sense of how front-loaded Endgame is until its second weekend, but note the film's stellar word-of-mouth.

"It's a can't-miss movie," says Disney distribution chief Cathleen Taff.

The tentpole, a direct sequel to Infinity War and a culmination of the previous 21 titles in the Marvel Cinematic Universe, is benefiting from a coveted A+ CinemaScore from audiences, glowing reviews and a staggering 82 percent recommend on PostTrak. It doesn't face any real competition for two more weeks until Detective Pikachu opens May 10.

Domestically, Endgame is expected to sail past $750 million by the end of its run, if not more. The top earners to date in North America are Force Awakens ($936.7 million), Avatar ($760.5 million), Black Panther ($700.1 million) and Infinity War ($678.8 million) and Titanic ($659.4 million), not adjusted.

"Avengers: Endgame burst onto the scene shattering pretty much every record that ever existed," wrote one studio rival in a box office note. "Avengers played best everywhere, from Maine to Maui and all places in-between."

Adds Dergarabedian, "Who knows, could $3 billion even be a possibility? Less than a week ago, we though it would be impossible for Endgame to open to more than $300 million or make a billion globally in its debut. Nothing is off the table when it comes to this film."