Fox’s signature musical reality series returned to the air with a bang for its fifth season last week, ending any speculation that the “Idol” premiere would be down now that viewers have already crowned four seasons’ worth of winners.

But the initial wake of the returning “Idol” apparently has hit some shows harder than others and done little to ensure who will win the season in the adults 18 to 49 demographic.

“Idol’s” two-hour premiere last Tuesday from 8 to 10 p.m. scored a 15.3 rating in adults 18 to 49-its second-highest performance in the demo ever-and garnered 35.5 million total viewers, according to Nielsen Media Research. Those ratings also included Nielsen’s live-plus-same-day sampling for DVR viewing.

If last season is a guide, Fox’s first-quarter offerings, led by “Idol” and the fifth season of real-time drama series “24,” will propel the network to the top spot in adults 18 to 49, despite having posted less competitive ratings in the fall.

This year Fox is still in “very good shape” when it comes to the demo ratings race, said Shari Anne Brill, VP and director of programming for Carat. But even if the network’s strong ratings continue, Fox is not certain to come out on top.

“They have a very good chance of winning [adults] 18 to 49 for the season, but ABC does have the Super Bowl and the Oscars,” she said, noting that CBS has been a consistent demo winner for much of the season so far.

“It could still be a three-way race,” she said.

Though the outcome of the season’s ratings race is uncertain, “Idol’s” premiere performance was assuredly strong.

“Idol’s” demo rating last week was its second-highest ever, rating behind only its season two finale in May 2003. It was also the highest-rated episode of entertainment programming on all six of the major networks since NBC aired the season finale of “Friends” in May 2004.

“Idol” was also up 9 percent in the demo and 6 percent in total viewers from its premiere last season. Against “Idol,” ABC’s 8:30 p.m. (ET) Tuesday comedy “Rodney” declined to a 1.5 in adults 18 to 49, dropping 42 percent from its previous week’s rating and hitting a new series low. NBC’s “Fear Factor” also suffered against the talent show, hitting its lowest demo rating with a 1.8 and dropping to fifth place in the time period behind The WB’s “Gilmore Girls,” which stayed even for the week with a 2.5 despite the onslaught of “Idol,”as was CBS’s “NCIS,” which held its 4.0 rating for the week.

It has been a good midseason overall for Fox, helped by its fifth-season premiere of “24”, a two-hour special Jan. 15. Boosted by a 13-minute NFL overrun, the first hour of “24” scored a 7.3 in the demo, its best performance ever. In its second hour, however, “24” scored a 6.7 and was eclipsed by ABC’s “Desperate Housewives,” which won its time period and the night with a 9.5.

Despite “Housewives,” “24” gave Fox its best Sunday night performance in the demo since last year’s Super Bowl.

Another two-hour block of “24” on Monday, Jan. 16, went head to head with NBC’s Golden Globes telecast. Scoring a 5.6 in its first hour, at 9 p.m. the series’ 6.3 in the demo tied the Globes as the highest-rated program for the night.

“Idol,” which begins its season with footage from its first set of auditions in various host cities, provides viewers with a contrast to the format used in later episodes, when the audience sees only the most accomplished singers and votes on its favorite performers, Ms. Brill said. “The audience shows always did well,” Ms. Brill said. “The best of the worst seems to carry as much appeal as the best of the best.”

The premiere of “Idol’s” one-hour Wednesday episode Jan. 18 at 8 p.m. also outdelivered its performance from last season, growing 15 percent in the demo to a 13.0. Capitalizing on “Idol,” Fox programmed a special preview of its debuting Monday night reality series “Skating With Celebrities” for the post-“Idol” 9 p.m. spot on Wednesday, which paid off for the network. “Skating,” which moves to its regular 8 p.m. time slot today, scored a 7.2 in adults 18 to 49, doubling Fox’s season-to-date time period average.

But that wasn’t enough to overtake the time period leader, ABC’s “Lost,” which won the 9 p.m. hour with an 8.0.