Fox’s once untouchable ratings powerhouse “American Idol” continues to show signs of vulnerability. The singing competition was beaten in the Thursday night ratings by CBS’s hit sitcom “The Big Bang Theory” for the third week in a row, TVbytheNumbers.com reports.

From 8-8:30 p.m., "Big Bang" had a 5.4 average rating in the 18-49 demographic while "Idol" settled for a 5.0 in the time slot. For the bottom of the hour, CBS’s "Rob" scored a 3.4 while "Idol" rose to a 5.8.

“American Idol’s” 5.4 average for the overall hour was a season low, down a tenth of a point from last week, in Nielsen overnights. Some adjustments are still forthcoming.

Fox did win the night overall in the 18-49 demo, averaging a 3.9 rating to CBS’s 3.5, ABC’s 2.5 and NBC’2 1.5. CBS came out on top in total viewers, averaging 14.1 million to 12.0 million for Fox, 7.4 million for ABC and 3.6 million for NBC.