Jan. 14 (Bloomberg) -- “American Idol,” the most-watched show on U.S. television, attracted 10 percent fewer viewers in the opening of its eighth season on News Corp.’s Fox network, ratings data show.

The twice-a-week singing contest won its time slot with 30.1 million viewers and a 15.8 rating over two hours last night, according to Nielsen Co. data supplied by competitors today. Fox had more than triple the rating of its nearest rival CBS Corp. in the 18-to-49 age group that advertisers target.

The show has given Fox the biggest audience in that segment the past four seasons and last year propelled the network to an overall ratings win. CBS, hobbled by the writers strike in 2008, could reclaim that title this year. Last season, “Idol” produced about $750 million in 2008 revenue for News Corp. according to UBS AG analyst Michael Morris.

“It’s still a powerhouse, even with ratings 5 or 10 percent lower,” Aaron Cohen, an advertising buyer at New York- based Horizon Media Inc., said in an interview before the results were released.

The opening night of “Idol,” in which the judges Simon Cowell, Randy Jackson, Paula Abdul and new panelist Kara DioGuardi vetted contestants in Arizona, had an 11.6 average rating for the night in 18- to 49-year-olds, Nielsen data show.

The number of viewers in that age group fell to 15.4 million from 18.1 million a year earlier.

A ratings point equals 1 percent of the potential audience. Last year’s debut had an initial 18.6 rating among all TV households.

Smallest Since 2004

The audience for last night’s show was the smallest since 28.6 million tuned in for opening night in January 2004, according to Nielsen. The 2008 debut attracted 33.5 million.

Heading into “Idol” season, show producer Ken Warwick predicted the contest would be unlikely to match season seven’s audience tally, when it averaged 27 million to 28 million viewers a night, according to Nielsen.

Still, Fox is confident the show will drive a fifth- straight ratings win, Preston Beckman, executive vice president of strategic programming, said in an interview last week.

“When the dust settles, will we be tops in 18-49 for a fifth year?” Beckman asked. “Absolutely.”

News Corp., controlled by Rupert Murdoch, fell 58 cents, or 6.7 percent, to $8.11 at 4 p.m. New York time in Nasdaq Stock Market trading. Merrill Lynch & Co. lowered its rating on the stock today to “neutral” from “buy,” citing economic concerns. The shares fell 56 percent last year.

To contact the reporter on this story: Andy Fixmer in Los Angeles at afixmer@bloomberg.net

Last Updated: January 14, 2009 16:02 EST