Though its ratings were lower than many had guesstimated, two nights of solid, winning numbers from Fox’s “The X Factor” were the deciding factor in premiere week, which saw the surging net kick off a season as the leader among young adults for the first time in its history.

CBS had a strong week as well, with Ashton Kutcher’s debut on “Two and a Half Men” standing as the dominant No. 1 program and the net’s early-week dramas performing well. It finished a close second in 18-49, a solid first in 25-54 and rolled as usual in total viewers.

ABC ran third, on par with last year, while fourth-place NBC was down about 7%.

Biggest story of premiere week had to be the comedy comeback, with half-hour laffers accounting for nine of the frame’s top 10 nonsports shows in 18-49, according to Nielsen. And from Monday through Thursday, it was new and returning comedies that were the clear standouts of each night.

Premiere week, of course, typically comes with some asterisks — including expanded episodes of hit shows like ABC’s “Modern Family” and “Grey’s Anatomy” and CBS’ “How I Met Your Mother” and “The Big Bang Theory.” Fox and NBC, on the other hand, stuck mostly to their regular skeds.

And in the case of CBS and Fox, their early-evening Sunday NFL overruns regularly skew year-to-year comparisons in the fall. CBS, for example, enjoyed a lengthy overrun a year ago, and was down 6% in 18-49 this year without it; Fox, on the other hand, saw its late-afternooon NFL action spill roughly 30 minutes into primetime this year, contributing to its 31% gains vs. the opening frame of 2010.

Another factor that has to be considered is the increased penetration level of digital video recorders, with 42% of the Nielsen sample households owning DVRs compared with 38% a year ago. The “live-plus 7” numbers, which include same-week DVR playback, are expected to again be bigger than last fall — based on preliminary “live-plus 3” numbers.

Looking at the Nielsens for the week of Sept. 19-25, Fox led in adults 18-49 with a 3.4 rating/9 share in adults 18-49, followed by CBS (3.1/8), ABC (2.8/8) and NBC (2.6/8). CBS held the edge over Fox in adults 25-54 (4.1/10 vs. 3.6/9) and drew 2 million total viewers more than its closer pursuer, ABC (12.1 million to 9.9 million).

Thanks mostly to Fox’s healthy gains, the Big Four were up vs. premiere week a year ago in 18-49 rating (a combined 11.9 vs. 11.5) and total viewers (39.5 million vs. 39.3 million).

Fox was strongest at midweek, with “New Girl” bowing to winning demos on Tuesday (4.8/12 in 18-49, 10.28m). It built on its lead-in from “Glee” (4.0/11, 9.21m) and became the net’s top-rated live-action fall comedy debut in 10 years; for its part, “Glee” dropped out of the top 10 after kicking off its second season a year ago as the No. 1 entertainment series in 18-49.

“The X Factor” bowed to solid but underwhelming scores on Wednesday (4.4/12, 12.49m) and Thursday (4.3/12, 12.51m), lifting the net to 18-49 victories on both nights even though it was not close to “American Idol” ratings territory. The Simon Cowell reality show provided Fox with significantly stronger showings in premiere week than a year ago, lifting Wednesday by 52% and Thursday by 79%.

CBS, which has now won premiere week in 25-54 and total viewers for three straight years, was paced by series highs for “Two and a Half Men” (10.7/25 in 18-49, 28.74m) — the top scores for a regular comedy on any net in six years — as well as the preview of “2 Broke Girls” that followed it (7.1/16, 19.37m).

Also Monday, “How I Met Your Mother” was up vs. last year (5.2/13 in 18-49, 12.22m for its 8:30 p.m. seg), as was Thursday’s “The Big Bang Theory” (5.1/14, 14.94m at 8:30).

CBS saw its two “NCIS” shows grow year to year, with the original the week’s No. 1 drama (4.3/12, 19.96m), and “Criminal Minds” grew on Wednesday (4.1/10, 14.14m). Among new dramas, “Unforgettable” led its Tuesday hour (2.9/8, 14.09m) and the net did OK with its bows of “Person of Interest” on Thursday (3.1/8, 13.33m) and “A Gifted Man” on Friday (1.4/5, 9.45m).ABC saw “Dancing With the Stars” return noticeably cooler (4.0/10, 19.03m) but the 10 p.m. dramas airing after it both nights — “Castle” and “Body of Proof” — held up well. Net saw similar results on Sunday, when the new “Pan Am” (3.1/8, 11.06m) impressed behind a declining “Desperate Housewives.”

ABC’s best night was Wednesday, where “Modern Family” hit a series high (6.2/15, 14.52m), providing a good lead-in for new soap “Revenge” (3.3/9, 10.02m) — a big improvement over last year’s “The Whole Truth.”

NBC had a pretty quiet premiere week, with its best results coming Thursday with “The Office” (3.9/10 in 18-49, 7.64m) leading into new comedy “Whitney” (3.3/8, 6.84m). Also, new Wednesday comedy “Up All Night” fared OK in its regular-slot bow (2.4/7, 6.05m).

“Sunday Night Football,” meanwhile, was the week’s No. 2 show (8.3/21, 20.36m), up vs. last year.

Not faring so hot were the bows of 10 p.m. dramas “The Playboy Club” on Monday (1.6/4, 5.02m) and “Prime Suspect” on Thursday (1.8/5, 6.05m).

CW was led by Thursday’s “Vampire Diaries” (1.2/3, 2.52m) and “Secret Circle” (0.9/3, 2.12m), both down vs. their early premieres.

Though it was broadcast’s big week, cable did claim three of primetime’s top 25 shows in adults 18-49: “Monday Night Football” at No. 9 (4.8/13), the Charlie Sheen “Comedy Central Roast” at No. 22 (3.8/9, 6.43m) and “Jersey Shore” at No. 24 (3.5/9, 6.49m).