The pre-season debut of NBC’s Thursday comedy block got off to a slow start last night, leaving the net a distant third among young adults for its two hours. Fox won the night with week-to-week gains for “The X Factor.”

According to preliminary national estimates from Nielsen, NBC opened Thursday with a “SNL Primetime Election Special” (1.6 rating/5 share in adults 18-49, 5.2 million viewers overall), which drew less than one-third of what a similar special premiered to four years ago (in early October) and ran third among the Big Four in its timeslot.

It was followed by the season premieres of returning comedies “Up All Night” (1.3/4 in 18-49, million viewers overall), “The Office” (2.1/6 in 18-49, 4.3 million viewers overall) and “Parks and Recreation” (1.7/5 in 18-49, 3.5 million viewers overall) — all of which paled in comparisons to their year-ago bows.

“The Office” wasn’t expected to approach its level of last year (4.0 rating in 18-49 during premiere week), which was the start of the post-Steve Carell era, but it kicked off its ninth and final season even lower than last May’s finale (2.3 demo rating). By comparison, “Parks and Recreation” held up better (1.7/5, 3.5 million viewers overall), down 19% from its post-“Office” premiere a year ago but matching its season finale from May.

“Up All Night” drew a 2.4 demo rating for its series premiere on a Wednesday a year ago.

Closing out the night was “Rock Center with Brian Williams” (1.1/3 in 18-49, 4.1 million viewers overall).

At Fox, “The X Factor” (3.4/11 in 18-49, 9.9 million viewers overall) logged week-to-week gains (as it also did this week on Wednesday) and stood as Thursday’s No. 1 show in all key categories. It was followed by “Glee” (2.9/8 in 18-49, 7.4 million viewers overall), which retained most of its week-ago season premiere score but couldn’t keep up with its lead-in, as it did last week.

CBS aired back-to-back repeats of “The Big Bang Theory” (2.0/7 in 18-49, 7.8 million viewers overall at 8 and 2.4/7 in 18-49, 9.2 million viewers overall at 8:30) and “Two and a Half Men” (1.8/5 in 18-49, 7.4 million viewers overall at 9 and 1.7/5 in 18-49, 6.8 million viewers overall at 9:30), and capped its night with a repeat of “Person of Interest” (1.3/4 in 18-49, 6.6 million viewers overall).

At ABC, “Wipeout” (1.4/4 in 18-49, 4.4 million viewers overall) was up from last week, and was followed by repeats of May’s season finales of “Grey’s Anatomy” (1.3/3 in 18-49, 4.2 million viewers overall) and “Scandal” (1.0/3 in 18-49, 3.4 million viewers overall). The net’s numbers could come down some due to a preemption in Charlotte for an NFL game.

Preliminary 18-49 averages for the night: Fox, 3.1/9; CBS, 1.8/5; NBC, 1.5/4; Univision, 1.4/4; ABC, 1.2/4; CW, 0.5/2.

In total viewers: Fox, 8.6 million; CBS, 7.4 million; ABC and NBC, 4.0 million; Univision, 3.3 million; CW, 1.3 million.