The post-Simon Cowell era of “American Idol” opened to predictably lower ratings Wednesday night for Fox. Show dominated the competition as usual but lagged last year’s preem by 18% in young adults and 13% in total viewers.

Fox, which sees this season of primetime’s No. 1 series as a reboot, was bracing for declines. Not only is the singing competish entering its 10th season and airing on new nights, but it’s seeing its biggest onscreen turnover yet with the exit of the popular Cowell.

Future weeks, of course, will tell whether Wednesday’s premiere audience was boosted by curiosity over new judges Jennifer Lopez andSteven Tyler or if the show might see even larger falloffs down the road. “Idol” typically sees its best numbers early in the season before drifting downward as the weather gets warmer and overall viewing levels fall.

Nielsen estimates that the two-hour season premiere of “American Idol,” which featured the debut of judges Lopez and Tyler, averaged a 9.7 rating/26 share in adults 18-49 while drawing 26.1 million viewers overall. It more than doubled the demo delivery of the night’s No. 2 show, ABC’s “Modern Family” (4.6/12 in 18-49, 10.94 million), which held up very well.

From 8 to 10 p.m., “Idol” beat the combined ABC-CBS-NBC competition by 56% in 18-49 (9.7 vs. 6.2) and by 4.5 million viewers overall (26.1 million vs. 21.6 million). Still, this is the lowest “Idol” premiere since its inaugural season, which aired in the summer. Last year’s opener on a Tuesday did an 11.8 demo rating and 29.9 million viewers overall, and the year before bowed comparably (11.7 and 30.4 million).

Wednesday’s 9.7 is the best for “Idol” since Feb. 9 — its ninth episode last season. It also towers over the previously highest rated entertainment telecast of the season — Fox’s “Glee” on Sept. 28 did a 5.9 “live plus same-day” demo rating — by a whopping 64%.

Looking closer at the numbers, “Idol” held up best among adults 50-plus (9.8 rating, down 1% year to year) and saw its biggest declines among persons 12-34 (7.9, down 25%).

Show made its Thursday debut last night, where it faced tough young-adult competish from comedies on CBS and NBC and “Wipeout” on ABC.

Elsewhere, ABC’s “Modern Family” and “The Middle” (2.6/8 in 18-49, 8.32m) didn’t seem too affected by “Idol,” rising 39% and 24%, respectively, vs. the first Wednesday “Idol” night a year ago. Rookie 8:30 p.m. laffer “Better With You” (2.1/6, 6.37m) dropped off a bit more than usual, and 9:30 p.m. show “Cougar Town” (2.4/6, 5.68m) saw a steep dropoff from “Modern Family.”

The night’s 10 p.m. drama battle produced a tight 18-49 race, with NBC’s “Law and Order: Special Victims Unit” (2.3/7 in 18-49, 6.97m) prevailing over ABC’s second week of “Off the Map” (2.1/6, 5.79m, down 5% from its preem) and the timeslot debut of “Blue Bloods” on CBS (2.0/6 in 18-49, 12.30m). The latter Tom Selleck drama, getting a four-week tryout on a higher-profile night, won the hour in adults 25-54 (3.1/8) and dominated in total viewers — logging the show’s best numbers since its series premiere on a Friday in September.

Earlier in the evening, CBS was a nonfactor with competition series “Live to Dance” (1.0/3, 5.08m) before picking up nicely at 9 p.m. with “Criminal Minds” (2.9/7, 12.02m), which was runner-up to “Idol” in most categories with its second half-hour.

NBC opened with gameshow “Minute to Win It” (1.6/4, 5.06m) before falling flat at 9 o’clock with the timeslot debut of rookie drama “Chase” (1.0/3, 4.35m).