Martial arts greats Jet Li and Jackie Chan united for the first time onscreen for “The Forbidden Kingdom,” a Hollywood martial arts-adventure film that predictably topped the North American box office with an estimated $20.9 million in ticket sales within its opening weekend.

The very first reunion of acclaimed and favored Chinese martial artists Jet Li and Jackie Chan drew hordes of moviegoers to theaters across North America, with ticket sales worth an estimated $20.9 million for the Friday to Sunday period, according to box office tracker Media By Numbers.

“The Forbidden Kingdom” is directed by Rob Minkoff (“Stuart Little”) from a script written by John Fusco, and co-stars Li Bingbing, Collin Chou, Michael Angarano and Liu Yifei. It tells the story of a faraway kingdom accessible only to the initiated, where Chan portrays a kung fu master with a taste for wine and Li a silent monk who seeks peace.

“Forgetting Sarah Marshall,” a comedy rated R for sexual content, language and some graphic nudity (most notably, a rare display of full-frontal male nudity), came in second, with an estimated $17.3 million. The film is directed by Nicholas Stoller and co-produced by Judd Apatow.

Starring Jason Segel (who also wrote the script), Kristen Bell, Mila Kunis and British comedian Russell Brand, the raunchy comedy follows in the tradition of Apatow success stories “Superbad,” “Knocked Up” and “The 40-Year-Old Virgin.”

“Prom Night,” the critically reviled thriller that debuted at the top of the box office last week, has now dropped two places to three, with additional earnings of $9.1 million. This brings its 10-day haul to $32.6 million.

“88 Minutes,” another critically reviled film, this time a psychological thriller starring Hollywood veteran Al Pacino as forensics expert Dr. Jack Gramm faced with a personal threat of assassination, ranked fourth, with $6.8 million. The film was made two years ago and has already been released on DVD in some territories.

Family film “Nim’s Island,” starring Abigail Breslin, Jodie Foster and Gerard Butler, follows at No. 5, with $5.7 million.

Another weekend release was “Expelled: No Intelligence Allowed,” which opened at No. 9 with about $3.2 million. The documentary focuses on intelligent design – a controversial hypothesis that some biological structures are too complex to have appeared merely through natural selection and evolution.