Martial arts icons Jet LI and Jackie Chan's first big screen collaboration has reaped big rewards at the U.s. box office, crashing in at number one.

Their new period action film, The Forbidden Kingdom, beat sex comedy Forgetting Sarah Marshall to the top spot with an opening weekend take of $20.9 million.

Forgetting Sarah Marshall came in a close second, ahead of former chart-topper Prom Night at three and Al Pacino's new thriller 88 Minutes at four.

Family film Nim's Island rounded out the top five, according to studio estimates.