And the box office champion this week is...

LOS ANGELES – Audiences weren't afraid of committing to "No Strings Attached," making it the No. 1 movie at the box office in its opening weekend.

The romantic comedy from Paramount Pictures earned an estimated $20.3 million, according to studio estimates released Sunday.

Ashton Kutcher and Natalie Portman star as friends who try to maintain a purely sexual relationship with each other, even as they find they're falling in love. The strong supporting cast includes Greta Gerwig, Mindy Kaling, Lake Bell and Kevin Kline.

Last week's No. 1 movie, Columbia Pictures' action comedy "The Green Hornet," dropped to second place with $18.1 million. It has now made $63.4 million total. Seth Rogen stars as the title playboy-turned-superhero, based on the 1930s radio show, with Taiwanese pop star Jay Chou as his sidekick, Kato.

Much of the success of "No Strings Attached" could be attributed to the pairing of director Ivan Reitman and Montecito Pictures, which brought the movie in at a budget of just $25 million, said Don Harris, Paramount's executive vice president of distribution.

He thought audiences were drawn to the chemistry between the co-stars: "Natalie Portman couldn't be any hotter, and I think this is the kind of movie Ashton Kutcher had hoped to make and be in during his career. They go well together."

Portman just won a Golden Globe award for her portrayal of a ballerina gone mad in "Black Swan," the weekend's sixth-highest grossing film, and for which she's likely to receive an Academy Award nomination Tuesday. She recently announced she's expecting a baby with her fiance, Benjamin Millepied, who choreographed "Black Swan" and has a cameo in it.

Part of the allure of "No Strings Attached" also comes from the humor, which Harris said was a little edgier than expected in a romantic comedy.

"This was always intended to be an R," he said. "That's why I think these two were interested in being in the movie."

"No Strings Attached" was marketed as the perfect date movie, with its slogan: "Can sex friends stay best friends?", said Hollywood.com analyst Paul Dergarabedian.

"It's a so-called 'chick flick' because of the subject matter, but it had appeal for both men and women with the whole idea of being 'friends with benefits,'" Dergarabedian said. "For it to do $20 million is fairly impressive, considering it's rated R and January's not a hotbed of huge grosses, unless you're 'Avatar' and you're holding over."

It was the 11th weekend in a row that was down from the same time the previous year, when James Cameron's behemoth "Avatar" was still dominating from its debut at the end of 2009.

While Dergarabedian expected 2011 could be a record year at the box office, with total grosses surpassing $11 billion for the first time ever, "the huge year hasn't kicked in yet."

"I don't see much light at the end of the tunnel until March, when 'The Adjustment Bureau' and other movies open," he said, referring to the upcoming thriller starring Matt Damon and Emily Blunt. "We could be looking at a pretty sizable down streak."

Estimated ticket sales for Friday through Sunday at U.S. and Canadian theaters, according to Hollywood.com. Final figures will be released Monday.

1. "No Strings Attached," $20.3 million.

2. "The Green Hornet," $18.1 million.

3. "The Dilemma," $9.7 million.

4. "The King's Speech," $9.2 million.

5. "True Grit," $8 million.

6. "Black Swan," $6.2 million.

7. "The Fighter," $4.5 million.

8. "Little Fockers," $4.4 million.

9. "Yogi Bear," $4.1 million.

10. "Tron: Legacy," $3.7 million.