In a season that has seen the half-hour comedy dominate primetime, two more half-hours opened to promising numbers last week. But keeping that early momentum may prove difficult.

CBS’ Rob Schneider comedy “Rob” and NBC’s Chelsea Handler skein “Are You There, Chelsea,” both of which exceeded expectations with their premieres, will now have to do battle with Fox’s “American Idol,” which is set to disrupt viewing patterns like it does every year at this time.

When they scheduled their midseason comedies last year, both CBS and NBC thought their shows would have two weeks to establish themselves before “Idol” bowed, but Fox moved up the start date of its megahit to this week — meaning the new comedies will have to work hard to hold onto viewers.

Still, neither CBS nor NBC could have asked for better bows for their laffers.

On Wednesday, according to Nielsen, “Are You There, Chelsea” averaged a 2.3 rating/6 share in adults 18-49 and 6.18 million viewers overall in the 8:30 p.m. half-hour — building on its lead-in from “Whitney” on its new night (2.0/6, 5.79m) and giving NBC its best regular, in-season score in the timeslot in about a year.

While not a great number, the 2.3 demo score is the second best of NBC’s six comedies last week (behind “The Office”) and gives the network something to build on. As for “Whitney,” it outpeformed the last five episodes of “Up All Night” to air in Wednesday’s leadoff hour.

One night later, CBS opened the night with huge numbers for “The Big Bang Theory” (5.3/15 in 18-49, 16.13m), which will finish as the week’s No. 1 live-action program in 18-49. And “Rob” retained a good 77% chunk of that lead-in (4.1/11, 13.47m), rating as the night’s No. 2 program in 18-49 and faring better than any show coming out of “Big Bang” since it moved to Thursday in September 2010 (“Bleep My Dad Says,” “Rules of Engagement” and “How to Be a Gentleman”).

Fox’s Thursday drama “The Finder,” meanwhile, should benefit from the return of “Idol,” which it will follow.

In its bow last week, the “Bones” spinoff settled for a 1.7/4 in 18-49 and 5.5 million viewers in the 9 o’clock hour — last among the Big Four and well behind dramas “Grey’s Anatomy” on ABC (3.8/10, 10.71m) and “Person of Interest” on CBS (series-best 3.2/8, 14.86m).

In the same hour, NBC’s “Up All Night” had a sluggish start in the 9:30 slot (2.0/5, 4.24m) behind “The Office” (2.9/7, 5.90m). Earlier, the returning “30 Rock” kicked off the night modestly (1.8/5, 4.47m), faring slightly better than “Community” in the timeslot.And closing the night, it was slow going for the Peacock’s “The Firm” (1.0/3, 4.23m), which placed fifth in 18-49 among English-language networks and fared no better than recent slot occupant “Prime Suspect,” which has been canceled.