LOS ANGELES (AP) _ CBS scored an upset over unbeaten NBC in the television Nielsen ratings as NBC benched ''The Cosby Show'' and the rest of its powerhouse Thursday night lineup for the New Year's football bowl games.

NBC tumbled before CBS during the week ending Jan. 4, placing only one of its regular shows, ''The Golden Girls,'' in the Top 10. NBC did win three places with football games.

The defeat was the first for NBC this television season, and it was CBS' first victory since the week ended Aug. 31.

CBS, led by ''Murder, She Wrote'' in first place, had four shows in the Top 10. ABC had two shows at the top.

''It would have been nice to show up here with an 18-0 record for the season, but 17-1 isn't bad,'' Brandon Tartikoff, president of NBC Entertainment, said Tuesday at the semi-annual meeting of the television press at the Century Plaza Hotel.

''Even though NBC lost its big Thursday night schedule, football helped them,'' David Poltrack, CBS vice president for research, said by telephone from New York.

Actually, ''The Cosby Show'' didn't do that well the week before. The show, usually first, was fifth. The program, however, was a rerun and was telecast Christmas night.

In addition, CBS took the ratings lead among the evening news shows away from NBC.

''Our research has shown that Dan Rather does better in the ratings during holiday weeks because people who normally are not home in time to watch the news pick CBS when they are home early,'' Poltrack said.

CBS was first in the A.C. Nielsen Co. survey with a rating of 16.8. NBC was a close second with 16.3 and ABC was third with 13.0. The networks say this means that in an average prime-time minute 16.8 percent of the television homes were tuned to CBS.

''This is certainly not a trend,'' Tartikoff said. ''From the outset we knew we would not have a perfect season. We expected to lose the week of the Super Bowl.''

CBS will telecast the Super Bowl on Sunday, Jan. 25.

NBC still has a hefty lead over the other two networks for the season to date. NBC has a rating of 17.9, CBS has 15.9 and ABC 14.1.

Here are the Top 10 shows for the week ending Jan. 4: ''Murder, She Wrote,'' CBS; ''The Golden Girls,'' NBC; NFL football game between Denver and New England, NBC; Fiesta Bowl football game between Penn State and Miami, NBC; CBS Sunday Movie ''At Mother's Request,'' Part I, CBS; Rose Bowl football game between Michigan and Arizona State, NBC; ''60 Minutes,'' CBS; ''Growing Pains,'' ABC; ''Dallas,'' CBS; ''Who's the Boss,'' ABC.

NBC's coverage of the Rose Bowl, which went into prime time on New Year's Day, was sixth in the ratings. But NBC's night game, the Orange Bowl featuring Oklahoma and Arkansas, placed 27th.

CBS also dominated the second 10 shows with six. CBS' new series ''Outlaws'' was 17th.

''The performance of 'Outlaws' is very significant in turning around our Saturday night fortunes,'' Poltrack said.

Here are the next 10 shows: ''Moonlighting,'' ABC; ''Amen,'' NBC; ''Knots Landing,'' CBS; ''My Sister Sam,'' CBS; ''227,'' NBC; ''Newhart,'' CBS; ''Outlaws,'' CBS; ''Kate & Allie,'' CBS; ''Hunter,'' NBC; ''Falcon Crest,'' CBS.

In the news shows ratings, CBS had 13.7, NBC 12.2 and ABC 10.9.