American Idol” may be down but it can still lead Fox to nightly victories, as it did on Thursday. The show’s two-hour auditions episode won its time period in key demos, though CBS’ “The Big Bang Theory” was easily the night’s No. 1 show and “Mom” followed it up with across-the-board series highs. And 10 p.m. dramas “Elementary” on CBS and “Parenthood” on NBC matched their season highs as they enjoy some weeks without firstrun ABC competition.

According to preliminary national estimates from Nielsen, “American Idol” averaged a 3.0 rating/9 share in adults 18-49 and 11.0 million viewers overall from 8 to 10 p.m., retaining nearly all of its hourlong demo score of the previous night (3.1 in the prelims, 3.2 in the nationals). This is the show’s best night two performance relative to night one in six years in 18-49, and the first time in 13 years (since its first season) that it drew more total viewers on its second night.

A year ago, the Thursday premiere of “Idol” averaged a 3.9/12 in 18-49 and 13.4 million viewers overall (on a night when “Big Bang Theory” aired a repeat), so last night’s episode was down about 20%. Fox has now won its opening two nights of “American Idol” in every season since 2003 (season 2).

CBS was led as usual by “The Big Bang Theory” (4.5/14 in 18-49, 17.8 million viewers overall), which posted its highest score since moving to Thursdays in late October and was the night’s No. 1 program in every category except female teens (“American Idol”). “Mom” (2.8/8 in 18-49, 12.3 million viewers overall) continues to show better retention than previous post-“Big Bang” comedies, hitting series highs in all key demos as well as total viewers.

Ratings went down from there as usual with “Two and a Half Men” (2.2/6 in 18-49, 10.0 million viewers overall), “The McCarthys” (1.7/5 in 18-49, 7.9 million viewers overall) and “Elementary” (1.4/4 in 18-49, 8.4 million viewers overall), though they all were on the high end of their range. In fact, both “McCarthys” and “Elementary” matched their season highs in 18-49.

At NBC, “The Biggest Loser” (1.3/4 in 18-49, 5.0 million viewers overall) rose a big 0.4 from its holiday-impacted performance of last week and matching its top demo score during the television season (its pre-season premiere did a 1.6). “Bad Judge” (0.8/2 in 18-49, 3.1 million viewers overall) and “A to Z” (0.5/2 in 18-49, 2.0 million viewers overall) were higher too, though these low-rated rookies are simply running out the clock at this point.

Nearing its series finale, “Parenthood” (1.4/4 in 18-49, 4.6 million viewers overall) tied with “Elementary” in 18-49 and matched the show’s top score in the demo in its last 29 firstrun episodes (since October 2013).

ABC had another low-rated Thursday with “The Taste” (0.9/3 in 18-49, 3.2 million viewers overall) and a repeat of “How to Get Away with Murder” (0.7/2 in 18-49, 2.6 million viewers overall), though the former managed to match its season high.

CW aired the movie “House at the End of the Street” (0.3/1 in 18-49, 1.3 million viewers overall)

Preliminary 18-49 averages for the night: Fox, 3.0/9; CBS, 2.4/7; Univision, 1.2/4; NBC, 1.1/3; ABC and Telemundo, 0.8/2; CW, 0.3/1.

In total viewers: Fox, 11.0 million; CBS, 10.8 million; NBC, 4.1 million; Univision, 3.1 million; ABC, 3.0 million; Telemundo, 1.8 million; CW, 1.3 million.