“House” and “Fringe” kept Fox on top in key demos on the opening Tuesday of the new season despite week-to-week declines opposite increased competition, which included perky premiere numbers for CBS’ drama combo of vet “NCIS” and newcomer “The Mentalist.”

ABC, meanwhile, was strong closing out the night with another expanded “Dancing With the Stars,” but its family gameshow “Opportunity Knocks” saw a sluggish bow in the opening hour. And NBC was competitive with its second week of “The Biggest Loser” and the season debut of “Law & Order: SVU.”

The CBS dramas turned in the hottest perfs of the night, with Simon Baker starrer “The Mentalist” holding onto nearly all of its potent “NCIS” lead-in among young adults. The 8-10 p.m. duo helped CBS rise 10% above its year-ago perf, according to Nielsen, and the net drew its largest Tuesday premiere aud since 2001.

Overall, the five broadcast nets saw their combined 18-49 rating (16.2) dip 6% below the opening Tuesday of a year ago (17.3), a reversal of Monday’s growth spurt when the nets collectively were up year-to-year by 8%. While CBS grew 10% Tuesday and ABC was up 3%, NBC eased 8%, Fox slid 16% and CW 20%.

Warner Bros. TV’s “The Mentalist,” in which Baker portrays a former TV psychic who uses his keen observational skills to solve crimes, averaged a 3.5 rating/9 share in adults 18-49 and 15.6 million viewers overall, finishing third in the 9 o’clock hour among 18-49 and second in both 25-54 (5.0/11) and total viewers. In a good sign, the show gained in key demos in its second half-hour.

It marked a 20% improvement in the 18-49 demo for the Eye over last year’s season preem of “The Unit” (which had to face “House”).

And leading into “The Mentalist,” the sixth-season premiere of “NCIS” (3.6/10 in 18-49, 18.03m) jumped 13% above last year’s 18-49 premiere while setting records for an opener in both 25-54 (5.3/13) and total viewers. In 25-54, “NCIS” impressively pulled to within 1 share of “House” for the hour’s lead.

Closing things out, the seventh-season preem of “Without a Trace” (2.8/7 in 18-49, 11.28m) ran third in all key demos on its new night, coming in a bit below the 18-49 debut of “Cane” last year (3.0/8).

At Fox, “House” (5.1/14 in 18-49, 12.38m) won the night’s opening hour among young adults — and was the night’s top show in demos — but remained well off its early pace of last season, when it aired at 9. It was followed by rookie drama “Fringe” (4.1/10, 9.42m), which was down 20% week to week yet still retained a solid percentage of its “House” lead-in.

ABC’s family gameshow “Opportunity Knocks” (1.8/5 in 18-49, 6.56m) ran fourth in the 8 o’clock hour but built some in its second half-hour; it finished well below comedies “Cavemen” and “Carpoolers” in the hour a year ago. From 9 to 11 p.m., “Dancing With the Stars” (a slot-leading 4.7/12 in 18-49, 18.30m) was in line with its year-ago 90-minute Tuesday premiere, and edged out “NCIS” as the night’s most-watched program overall.

NBC’s two-hour installment of “The Biggest Loser” (3.0/8 in 18-49, 7.36m) matched its delivery on premiere Tuesday of last season. And at 10, “Law & Order: SVU” (3.8/10 in 18-49, 9.68m) was down 21% vs. a year ago and placed second to “Dancing” in demos.

At CW, “90210” (1.5/4 in 18-49, 2.94m) dropped week to week but was up 11% in 18-34 vs. “Beauty and the Geek” in the hour a year ago (2.0/6 vs. 1.8/5). Fellow rookie drama “Privileged” isn’t looking so hot, though, slipping to a 0.8/2 in 18-49 and 1.81 million viewers overall.