"American Idol" dropped by some 18 percent to 21.6 million viewers when the TV singing contest returned for its 11th season, according to early ratings figures on Thursday.

The drop was even bigger -- about 27 percent -- among viewers 18 to 49-year-olds, the audience group most coveted by advertisers, Nielsen figures showed.

But despite the fall-off, which Fox television executives said they had expected, the two-hour "American Idol" season premiere was still the most-watched show by a huge margin on U.S. television Wednesday night.

The 21.6 million viewers on Wednesday, compared to 26.2 million who watched the 2011 season premiere when new judges Steven Tyler and Jennifer Lopez made their first appearance.