NBC’s holiday reality series The Sing Off continues its impressive sophomore run. Last night, the a cappella singing competition (3.3/8 in adults 18-49, 9.4 million viewers) was up 18% from its series debut last Monday and up 22% from the show’s second airing on Wednesday to hit a series high. It posted NBC’s best 18-49 and total viewer numbers in the slot since Sept. 2009. Sing Off ran a solid second to CBS in the 18-49 demo from 8-10 PM, edging CBS veteran Rules of Engagement (3.4 vs 3.3) for No. 1 at 8:30 PM. Meanwhile, running distant fourth in the 8 PM hour was the other unscripted offering last night, ABC’s anemic Skating with the Stars (0.8/2), which matched last week’s rating. The network did only marginally better at 9 PM with the Mariah Carey: Merry Christmas special (1.1/3), which also finished fourth. Both NBC and ABC aired reruns at 10 PM, Chase (1.3/3) and Castle (1.2/3), respectively.

CBS’ fast-national numbers were probably inflated a bit by an NFL preemption in Baltimore. All series are currently running a notch above their performances last week: How I Met Your Mother (3.7/10, up 3%), Rules of Engagement (3.3/9, up 6%), Two and a Half Men (4.4/11, up 5%), Mike & Molly (3.7/9, up 9%) and Hawaii Five-O (2.9/8, up 4%). CBS, the only broadcast network, to offer all originals, won the night in adults 18-49 (3.5/9) and total viewers (11.6 million).

Fox aired an all-rerun lineup of House (2.4/6) and Lie to Me (1.9/5), whose numbers are also expected to go down in the finals due to NFL  airings in New York and Minneapolis. The CW also ran repeats.