Cast member Natalie Portman poses at the premiere of "No Strings Attached" at the Regency Village theatre in Los Angeles January 11, 2011. The movie opens in the U.S. on January 21. [Photo/Agencies]

Natalie Portman, already riding high during awards season with "Black Swan," took the No. 1 spot at the North American box office on Sunday for the first time in five years with her romantic comedy debut.

"No Strings Attached," in which she co-stars with Ashton Kutcher, sold about $20.3 million worth of tickets across the United States and Canada during the three days beginning January 21, distributor Paramount Pictures said.

The opening exceeded the modest expectations of the Viacom Inc unit, which said the film cost just $25 million to make. Portman and Kutcher play friends who enter into a sexual relationship with the titular proviso. Complications ensue.

Paramount said women accounted for 70 percent of the audience, and patrons aged under 25 gave it the best reviews in exit polls. Paramount said it was impossible to tell how the "Black Swan" buzz helped, but "everything helps," noted the studio's distribution president Don Harris.

The film was directed by "Ghostbusters" veteran Ivan Reitman who has not had a big hit since "Six Days, Seven Nights" in 1998. Kutcher's last big movie was "What Happens in Vegas," a 2008 romantic comedy with Cameron Diaz. Portman, 29, last led the box office in March 2006 when the thriller "V For Vendetta" grossed $26 million during its first weekend.

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Last weekend's champion, Columbia Pictures' 3D comic-book adaptation "The Green Hornet," slipped to No. 2 with $18.1 million. "The Dilemma," a Universal Pictures romantic comedy starring Vince Vaughn and Kevin James, was also down one, to No. 3, with $9.7 million in its second weekend. Their respective 10-day totals stand at $63.4 million and $33.4 million.

"No Strings Attached" was the weekend's only new wide release, in keeping with January's reputation as a graveyard for new films. Much of the focus is on the awards-season contenders, four of which were in the top 10.

That includes "Black Swan," in which Portman plays an unhinged ballerina. The Fox Searchlight melodrama slipped one to No. 6 with $6.2 million. It has earned $83.6 million to date, and is poised for further gains as the awards season enters the home stretch. Portman has won most of the bellwether awards leading up to Tuesday's announcement of the Academy Award nominations.

Elsewhere, Weinstein Co's royals drama "The King's Speech" was steady at No. 4 with $9.2 million. The film's Oscar chances received a surprise boost on Saturday when it was named best picture by the Producers Guild of America, a group whose picks usually go on to win the top Oscar. Its victory weakened the awards-season dominance of "The Social Network," which has just come out on DVD.

Paramount's hit Western remake "True Grit," largely overlooked during awards season, fell two to No. 5 with $8 million; its total stands at $138.6 million, easily a personal best for filmmakers Joel and Ethan Coen.

"The Fighter," also released by Paramount, rose two to No. 7 with $4.5 million; the boxing drama, buoyed by acclaim for Christian Bale's supporting turn as a boxer-turned-crackhead, has earned $73 million to date.

Columbia Pictures is a unit of Sony Corp. Universal Pictures, a unit of General Electric Co's NBC Universal. Weinstein Co is privately held. Fox Searchlight is a unit of News Corp.