CBS has opened the May sweep with a convincing primetime ratings victory in key demos, behind the still-potent combo of “Survivor” and “CSI,” according to preliminary nationals from Nielsen.

Although down some from the first full Thursday of last year’s May sweep, “Survivor: Exile Island” (5.8 rating/18 share in adults 18-49, 17.1 million viewers overall) and “CSI” (8.4/22 in 18-49, 26.9 million viewers overall) each dominated their hours and were the top-rated programs of the night in adults 18-49. And at 10, “Without a Trace” (5.5/15 in 18-49, 17.9 million viewers overall) was up slightly year-to-year, winning its hour.

NBC remained second on the night but fell further behind CBS, as each half-hour lagged its performance of a year ago. “Will & Grace” (2.8/9 in 18-49, 7.5 million viewers overall) was a distant second at 8, as were “My Name is Earl” (4.1/11 in 18-49, 9 million viewers overall) and “The Office” (3.8/10 in 18-49, 7.7 million viewers overall) in the 9 o’clock hour. In part due to weaker lead-ins, “ER” (5.2/14 in 18-49, 12.4 million viewers overall) fell more than 25% behind its pace of a year ago, when it beat “Without a Trace” in demos.

Fox struggled on the night with an hour of fresh “That ’70s Show” (third-place 2.2/7 in 18-49, 4.9 million viewers overall) and “The OC” (2.4/6 in 18-49, 5.4 million viewers overall). And ABC ran third at 9 with “American Inventor” (3.0/8 in 18-49, 7.2 million viewers overall) and was not a factor at 10 with “Commander in Chief” (1.9/5 in 18-49, 6.5 million viewers overall), which hit another series low.

Preliminary 18-49 averages for the night: CBS, 6.6/18; NBC, 4.0/11; Fox, 2.3/6; ABC, 2.1/6; WB, 1.7/5; UPN, 1.0/3.

In total viewers: CBS, 20.6 million; NBC, 9.3 million; ABC, 6.3 million; Fox, 5.2 million; WB, 4.0 million; UPN, 2.4 million.