ABC dove into a crowded singing pool with modest results Thursday as the debut of competition series ”Duets” finished a distant second in young adults among the broadcasters. Fox won the night with its return of ”So You Think You Can Dance,” but it was down sharply year to year.

According to preliminary national estimates from Nielsen, ”Duets” averaged a 1.7 rating/5 share in adults 18-49 and 6.7 million viewers overall from 8 to 10 p.m., with the show bowing less than 22 hours after Fox’s ”American Idol” wrapped its longest and lowest-rated season to date. In adults 18-49, ”Duets” performed nearly 50% better than the repeats ABC aired in the same two hours a year ago, but was well below the 2.4 demo rating delivered by the net’s Thursday premieres of last summer (returning ”Wipeout” and new ”Expedition Impossible”).

”Duets” was no match in demos for Fox’s ”So You Think You Can Dance” (2.4/8 in 18-49, 6.2 million viewers overall), with the Fox show prevailing in their head-to-head matchup by 41% in 18-49 (2.4 vs. 1.7) and by a larger 75% in the younger, 18-34 half of the demo (2.1 vs. 1.2); the gaps in both categories were larger in the final, 9:30 half-hour as the Fox show picked up steam while ABC’s stalled. ”Duets” was most competitive among women 25-54, with its 3.1 rating just a tick behind the Fox show. ”Dance” itself was down 32% from its bow on the same night a year ago, when it faced no original Big Four competition, but was up from the 2.0 it averaged for its finale in August.

Elsewhere on the night, ABC’s returning drama ”Rookie Blue” had a decent debut at 10 p.m. (1.4/4 in 18-49, 6.0 million viewers overall), down 13% from last year’s late-June bow, which enjoyed a stronger lead-in. The Canadian-produced cop drama finished a tick behind a repeat of CBS’ ”The Mentalist” in 18-49 (1.5/4, 7.8 million viewers overall) and outperformed the finale of NBC drama ”Awake” (0.9/2 in 18-49, 2.9 million viewers overall), which did bounce up some for its finale.

CBS’ all-repeat, regular lineup tied ABC’s all-original lineup in adults 18-49, led by ”The Big Bang Theory” (2.3/8 in 18-49, 8.3 million viewers overall) and ”Rules of Engagement” (1.7/6 in 18-49, 6.3 million viewers overall). Also for the net, ”Person of Interest” averaged a 1.4/4 in 18-49 and 7.8 million viewers overall in the 9 o’clock hour, and ”The Mentalist” wrapped things up at 10 with a timeslot victory among the broadcasters (1.5/4 in 18-49, 3.8 million viewers overall).

NBC struggled, generating its best numbers for a repeat of ”America’s Got Talent” at 9 p.m. (1.2/4 in 18-49, 3.8 million viewers overall); it was the fifth time in the last six nights that the show had aired on the net, either in original or repeat.

Preliminary 18-49 averages for the night: Fox, 2.4/8; ABC and CBS, 1.6/5; Univision, 1.2/4; NBC, 0.9/3;

In total viewers: CBS, 7.6 million; ABC, 6.5 million; Fox, 6.2 million; Univision, 3.2 million; NBC, 2.9 million; CW, 0.7 million.