Fox’s “American Idol” opened its 11th season Wednesday down significantly from last year, though its delivery among young adults still narrowly topped the combined delivery of ABC, CBS and NBC in the time period.

Preliminary Nielsen estimates show “Idol” averaging a 7.2 rating/18 share in adults 18-49 and 21.6 million viewers overall — down from the 9.8 demo score and 26.3 million viewers it premiered to a year ago and well off its bow if 2010 (11.8, 29.9 million). The roughly 25% decline vs. last year’s premiere marks the largest dropoff to date from the opening of one “Idol” season to the next.

Still, the only series telecast to fare better in both demos and total viewers this season was CBS’ season premiere of the rebooted “Two and a Half Men” with Ashton Kutcher (10.7 demo rating, 28.7 million viewers overall).

And “Idol” still towers over other music competition series. By comparison, NBC’s “The Voice” premiered last April to a 5.1/13 in 18-49 and 11.8 million viewers overall, and Fox’s “The X Factor” bowed four months ago with a 4.4/12 and 12.5 million viewers overall.

The gap certainly closed between “Idol” and its competition on Wednesday, as the other major networks were all up from 8 to 10 p.m. on the night of last year’s premiere. Whereas Fox led the combined ABC-CBS-NBC competition by about 55% last year, that was reduced to just 3% last night.

ABC was runner-up for the night in demos, led by “Modern Family” (5.0 rating/12 share in adults 18-49, 11.8 million viewers overall), which was on the low end of its range but still up nearly 10% year to year. Also for the net, “The Middle” placed second to “Idol” kicking off the night (2.6/7 in 18-49, 8.2 million viewers overall) and matched its year-ago score, while “Suburgatory” at 8:30 p.m. (2.5/6 in 18-49, 7.0 million viewers overall) and “Happy Endings” at 9:30 p.m. (2.5/6 in 18-49, 7.0 million viewers overall) were both up vs. the comedies the net was airing in these timeslots a year ago. And closing the night, “Revenge” (2.5/6 in 18-49, 7.6 million viewers overall) also gave ABC time-period gains, placing a solid second in the 10 o’clock hour; perhaps gaining some overflow “Idol” viewers, “Revenge” topped its previous five episodes among adults 18-34 (2.1/6).

CBS opened with an “NCIS” repeat (1.5/4 in 18-49, 9.1 million viewers overall) and then did a solid number at 9 p.m. with “Criminal Minds” (3.2/8 in 18-49, 12.8 million viewers overall), which was up 10% vs. its performance opposite “Idol’s” premiere last season. “CSI” then capped things for CBS with a victory at 10 p.m. (2.8/7 in 18-49, 11.8 million viewers overall); the episode was promoted as the first of the final two episodes featuring original castmember Marg Helgenberger, and next week’s hour is expected to perform even better.

NBC hung in there with its new comedy block of “Whitney” (1.5/5 in 18-49, 4.3 million viewers overall) and “Are You There, Chelsea?” (1.8/4 in 18-49, 4.3 million viewers overall), which did slightly better than gameshow “Minute to Win It” in the 8 o’clock hour a year ago. Similarly, “Harry’s Law” was a slight improvement at 9 (1.1/3 in 18-49, 6.4 million viewers overall), while vet “Law & Order: SVU” ran third at 10 (1.8/5 in 18-49, 6.4 million viewers overall) and was down vs. last year.

At CW, “One Tree Hill” (0.7/2 in 18-49, 1.5 million viewers overall) was down from its season premiere of last week, and it was followed by new reality show “Remodeled” (0.3/1 in 18-49, 0.6 million viewers overall for a repeat of the previous night’s episode).

Preliminary 18-49 averages for the night: Fox, 7.2/18; ABC, 3.0/8; CBS, 2.5/6; NBC, 1.5/4; Univision, 1.3/3; CW, 0.5/1.

In total viewers: Fox, 21.6 million; CBS, 11.2 million; ABC, 8.1 million; NBC, 5.7 million; Univision, 3.1 million; CW, 1.1 million.