Its task will get tougher in the coming weeks when it faces original episodes of “The Big Bang Theory” and “Grey’s Anatomy,” but “American Idol” had enough juice last night to win its two-hour time period and lead Fox to victory in key demos.

According to preliminary national estimates from Nielsen, the two-hour auditions episode of “American Idol” averaged a 2.6 rating/8 share in adults 18-49 and 10.4 million viewers overall from 8 to 10 p.m., down 13% from last week’s Thursday premiere and about 25% lower than its second Thursday of last season. It gained with each half-hour, peaking with a 2.9 rating in the 9:30 p.m. that equaled the combined tallies of CBS (1.5), ABC (0.9) and NBC (0.5).

Elsewhere, CBS’ “Mom” (2.4/8 in 18-49, 10.8 million viewers overall) was the only firstrun series that showed any pop. It retained nearly all of its young-adult lead-in from a repeat “The Big Bang Theory” (2.5/8 in 18-49, 12.1 million viewers overall), which swept the night’s opening half-hour, and was only 0.3 lower than last week when “Big Bang” was an original.

“Two and a Half Men” (2.1/6 in 18-49, 9.4 million viewers overall) and “The McCarthys” (1.5/4 in 18-49, 6.9 million viewers overall) were both down a couple of ticks from last week and placed second in their time periods. And after hitting a season high the previous week, “Elementary” (1.3/4 in 18-49, 8.1 million viewers overall) also dropped 0.2 and was the Big Four leader at 10 o’clock.

NBC opened with “The Biggest Loser” (1.1/4 in 18-49, 4.2 million viewers overall), which was down from last week’s above-average score and was followed by burnoff episodes of first-year half-hours “Bad Judge” (0.7/2 in 18-49, 2.8 million viewers overall) and “A to Z” (0.5/2 in 18-49, 1.7 million viewers overall). “Parenthood” (1.2/4 in 18-49, 4.1 million viewers overall) was down 0.2 from last week as it approaches its series finale.

ABC aired another two-hour installment of “The Taste” (0.9/3 in 18-49, 3.3 million viewers overall) and an encore “How to Get Away With Murder” (0.7/2 in 18-49, 2.6 million viewers overall) in advance of its Jan. 29 return.

CW aired the first edition of “The World Dog Awards,” which averaged a 0.4/1 in 18-49 and 1.9 million viewers overall and fared comparably to the “Critics Choice Movie Awards” airing in the same timeslot a year ago.

Things will heat up on Thursdays in the coming weeks, starting next week with the series premiere of “Backstrom” following “American Idol” on Fox. One week later, ABC’s “TGIT” lineup of dramas return with new episodes, and on Feb. 5 comes the timeslot debut of “The Blacklist” on NBC.

Preliminary 18-49 averages for the night: Fox, 2.6/8; CBS, 1.8/6; Univision, 1.3/4; NBC, 0.9/3; ABC, 0.8/3; Telemundo, 0.7/2; CW, 0.4/1.

In total viewers: Fox, 10.4 million; CBS, 9.2 million; NBC, 3.5 million; Univision, 3.3 million; ABC, 3.1 million; CW, 1.9 million; Telemundo, 1.5 million.