The era of mirror balls, shiny polyester, chest hair and coked-up morons flooded into the living rooms of millions during disco night on “American Idol.”

It was “Studio 57” all over again, as judge Kara DioGuardi might imprudently say.

While no one ever accused the disco era (roughly 1975-1980) of generating great melodies or even particularly memorable songs, it was certainly not the most boring span, either.

The genre offered Idol contestants a chance at dealing with songs that had actual grooves. For some that promised to be a good thing; for others, not so much.

The seven remaining singers had extra pressure this week following the “judges save” that resurrected the slim hopes of Matt Giraud. Regrettably, that meant two Idols would be voted off this week.

Kicking off the night was Lil Rounds, who attacked the stage like she was singing for her life with Chaka Khan’s “I’m Every Woman.” The very fact she didn’t pick the expected Donna Summer track was already a revelation for Rounds. She pumped up the crowd effectively, but her voice couldn’t follow suit. Weeks ago she seemed to have amazing vocal control, yet the last few shows have proved increasingly screechy. No matter what song she performs or arrangement she delivers, Rounds now comes across as a desperate wannabe.

On the other hand, Kris Allen took a stylistic riffle through the Jack Johnson playbook and executed it beautifully. Allen picked the unlikely Donna Summer track “She Works Hard for the Money,” stripping it down to just bass, percussion and his own acoustic guitar. While it easily could have been mistaken for something on the “Curious George” soundtrack, it was all kinds of effective. At the beginning of the season I predicted Allen would be a final three contestant, and I’m sticking with it.

Of course, I also predicted Danny Gokey would join him in that final trio. Now I’m not so sure. Gokey has the best vocal tone of any of the remaining male contestants, but he just hasn’t quite put the whole performance thing together. He chose the excellent “September” by the undeniably awesome Earth, Wind & Fire. But his uncomfortable and somewhat forced stage presence didn’t exactly scream “marquee draw.” In fact, he struck me in certain respects like Taylor Hicks — who is currently residing in the “where-are-they-now?” file.

Less awkward but still fumbling with the “stage appeal” aspect was Allison Iraheta. Donna Summer was once again invoked with the riff-oriented “Hot Stuff,” albeit through a half-time arrangement that all but extinguished the disco “heavy on the one” beat. Iraheta’s voice appeared strong and distinguished enough to win the competition, but the 16-year-old has yet to unveil much personality. She never seems able to come up with anything endearing or clever to say when given an opportunity. Chalk it up to a lack of life experience?

Not lacking personality — at least musically — was Adam Lambert. This season’s undisputed master of making unpredictable, avant-garde song choices, Lambert ditched disco queen Summer and headed straight for Hawaiian balladeer Yvonne Elliman. True, Elliman’s “If I Can’t Have You” was a number one track off the “Saturday Night Fever” soundtrack. Yet you would never have known the song was attached to that era, given the stripped-down, non-disco spin Lambert supplied. His castrato-ranging cover didn’t exactly bring the house down, but it did assure Lambert would keep heading toward his inevitable spot in the finals.

Speaking of “Saturday Night Fever” — or “Saturday Night Live,” as DioGuardi referred to it. (The new judge is slowly earning her way toward a slot alongside George Bush and Yogi Berra in the Hall of Confused Phrasing.) Matt Giraud chose what is disco’s quintessential track: “Stayin’ Alive” by The Bee Gees. Not exactly thinking outside the box. Giraud flashed some stage charisma in his Timberlake-ish getup. He sounded solid. He moved well. Let’s be real, though. He’s simply got no star mojo.

Dressed in a pink shirt and striped blazer that screamed “yachting weekend,” Anoop Desai completed the trifecta of Mrs. Summer’s hits with his version of “Dim All the Lights.” Did anybody else notice he sang to a prerecorded track instead of utilizing the live band? If that didn’t suck enough energy out of the arrangement then the lethargic tempo choice did the trick. Like Iraheta, Giraud and Rounds before him, Desai also struggled in vain to hit the final note of the performance, resulting in a truly forgettable end to an already fairly forgettable evening.

Well, at least this week the show came in on time.

Bottom three prediction: Anoop Desai, Lil Rounds and Matt Giraud.

Losers: Lil Rounds and Anoop Desai.