NBC’s ratings for Olympics coverage were down Thursday night, with the three hours of prime-time coverage slipping to a 6.8 average rating in the key 18-49 demo in Nielsen overnights.

TVbytheNumbers.com reports that preseason NFL action in a number of local markets tweaked the numbers, which are likely to be subject to more than the usual adjustments when final figures come in later.

Using the available overnight figures, NBC’s 6.8 represented a 39% drop from the 11.2 the network drew for Thursday night of the previous week. The number was also down 6% from the comparable night four years ago in Beijing.

On the other networks, one show managed to break a 2 rating in the 18-49 demo: CBS’s “Big Brother” delivered a 2.1, up 5% from a week earlier.

ABC put up a fresh episode of “Wipeout” at 9 p.m., which delivered a 1.7 in 18-49, up 21% from last week’s series low. A new ep of “Rookie Blue” at 10 p.m. was even with its previous fresh installment with a 1.5.

Everything else on the major broadcast nets was repeat programming.

The overnights for prime time as a whole show NBC in front with its 6.8 in viewers 18-49, followed by CBS (1.7 average), ABC (1.6), Univision (1.5) and Fox (1.0). In total viewers it’s NBC on top with 22.4 million, followed by CBS (5.8 million), ABC (5.1 million), Univision (4.0 million) and Fox (2.7 million).