“Big Bang Theory” came back to Earth a bit in its first head-to-head encounter this season with Fox’s “American Idol,” but the hot comedy had no problem topping the music contest as CBS won Thursday — its first demo victory on the opening night of the February sweep since 2006.

Also looking strong Thursday were CBS’ “Person of Interest” and ABC’s “Scandal,” while NBC’s “Do No Harm” had an anemic premiere. And NBC’s “30 Rock” hit a season high with its series finale, placing third in the night’s opening hour.

According to preliminary national estimates from Nielsen, CBS’ “The Big Bang Theory” was the night’s No. 1 program in all key categories, including adults 18-49 (5.3 rating/15 share) and total viewers (17.5 million). It was down about 15% from its record-setting level from earlier this month, but it bested “American Idol” head-to-head by a significant 30% margin in 18-49 (5.3 to 4.1).

From there, “Two and a Half Men” (3.9/11 in 18-49, 13.7 million viewers overall) placed second to “Idol” in the 8:30 p.m. half-hour, and sophomore drama “Person of Interest” (3.2/9 in 18-49, 15.6 million viewers overall) put the Eye back on top at 9 o’clock, beating out ABC’s “Grey’s Anatomy” by 14% in 18-49 and by a hefty 35% in 25-54 (4.6/11 to 3.4/8) and total viewers.

And at 10, “Elementary” (2.2/6 in 18-49, 10.8 million viewers overall) ran second in 18-49 and first in 25-54 (3.3/8) and total viewers. The first-year crime drama will get additional exposure on Sunday when it follows CBS’ coverage of the Super Bowl.

For Fox, “American Idol” (4.5/13 in 18-49, 13.7 million viewers overall) ran nearly neck and neck in 18-49 with CBS for the whole 8 o’clock hour, declining 13% from last week when it didn’t face the comedies; through its first 10 hours this season, “Idol” is within 10% of its year-ago 18-49 average (5.5 vs. 6.1). And at 9, “Glee” (2.1/6 in 18-49, 5.5 million viewers overall) fell off nearly 20% from last week’s above-average score.

At ABC, a special “Shark Tank” repeat kicked off the night (1.1/3 in 18-49, 4.6 million viewers overall), followed by a below-average “Grey’s Anatomy” (second-place 2.8/7 in 18-49, 8.8 million viewers overall). And after a week off, “Scandal” returned strong at 10 p.m. (2.7/8 in 18-49, 8.1 million viewers overall). If it holds in the nationals, this would be the show’s second best score to date, its best retention yet of “Grey’s Anatomy” as well as its biggest 18-49 victory over “Elementary.”

NBC said goodbye to critical darling “30 Rock” (1.9/5 in 18-49, 4.8 million viewers overall), which rose about 35% week to week to hit a 10-month high. It was followed by back-to-back originals of “The Office” in its regular slot at 9 (2.1/5 in 18-49, 4.4 million viewers overall) and then at 9:30 p.m. (1.9/5 in 18-49, 4.0 million viewers overall). But even with an above-average timeslot performance for the net at 9:30, the Jekyll & Hyde drama “Do No Harm” opened to a tiny 0.9/3 in 18-49 and 3.1 million viewers overall, albeit in a tough spot opposite hit dramas on both ABC and NBC. NBC hadn’t done much better in the hour earlier this season with newsmag “Rock Center,” but it’s unlikely to tolerate a consistent sub-1 demo rating in the hour leading into the late local news on its affiliates. A year ago at this time, the net pulled “The Firm” when it did a 0.8 on the opening night of the sweep.

CW saw a good showing by “The Vampire Diaries” (1.3/4 in 18-49, 2.6 million viewers overall) despite the double-barrel competition from the CBS comedies and “American Idol.” And at 9, “Beauty and the Beast” (0.7/2 in 18-49, 1.7 million viewers overall) similarly held up well.

Preliminary 18-49 averages for the night: CBS, 3.4/9; Fox, 3.3/9; ABC, 2.2/6; NBC and Univision, 1.6/4; CW, 1.0/3.

In total viewers: CBS, 14.0 million; Fox, 9.6 million; ABC, 7.2 million; NBC, 4.1 million; Univision, 4.0 million.