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NEW YORK (AP) — The packed dance audience at New York City Center wasn’t missing a trick.

Just before the lights went down for the second act of Twyla Tharp’s new program Wednesday night, some in the crowd spotted the 81-year-old choreographer sneaking into her seat, small and lithe, with a bob of gray hair — and unmistakable to dance fans. There was a round of sustained cheers.

If the adoration seemed intense, take note that this crowd had just watched her dancers perform “In the Upper Room,” Tharp’s breathtaking 1986 classic that sends dancers to the outer reaches of their capabilities.

Breathtaking is an apt description in more ways than one. Audience members literally gasp, but one imagines the dancers do so even more, in the wings, in the (very) brief breaks between entries and exits. That they manage to find enough breath is almost miraculous — and explains their wide grins at curtain calls.

What are they thinking? It seems the dancers — and there have been many, from different companies, over 36 years — are delighted both with performing the work, and having survived it. There is no doubt that Tharp’s fiendishly difficult choreography, set to the propulsive music of Philip Glass, is a test of endurance that only the best dancers can even contemplate tackling. But there is, always, an undercurrent of joy and exhilaration. Tharp’s masterpiece is a work that virtually nobody tires of seeing again and again — and almost an addiction for some dance fans (guilty as charged).

For this current iteration, which lasts through Oct. 23 at City Center, Tharp has paired “In the Upper Room” with another well-known and very different work, her 1982 “Nine Sinatra Songs.”

And she has brought together an excellent ensemble of 17 dancers from a variety of companies, including New York City Ballet, American Ballet Theater, Martha Graham and Alvin Ailey, plus former dancers from Miami City Ballet and San Francisco Ballet among others. It’s a collection of veterans and some in earlier stages of their careers. Several dancers were on the retirement path; one, Jada German, recently graduated from Juilliard.

In “Upper Room,” the curtain rises on a stage filled with fog, through which dancers suddenly appear — “out of nowhere,” Tharp has said. The nine sections bring different groups on and off — five dancers, 10 dancers, six dancers (there are a total of 13).

First up are what Tharp calls the head “stompers” — female dancers in white sneakers. In this production the honors were done by willowy Kaitlyn Gilliland, formerly of NYCB, and Stephanie Petersen, formerly of ABT.

There are also three standout lead dancers in bright red pointe shoes and anklet socks: Jeanette Delgado, German, and current ABT principal Cassandra Trenary.

The costumes are key: Norma Kamali’s ensembles morph as the 40-minute whirlwind of movement progresses. Black-and-white striped pajama-style outfits peel off, first tops and then bottoms, to reveal bright red leotards underneath. And some of the male dancers — Lloyd Knight, Richard Villaverde and Reed Tankersley — have the job of shedding their shirts midway and displaying, not least through sweat, just how hard everyone is working (very hard).

In the second-act “Nine Sinatra Songs,” Tharp focuses on couples, and more specifically relationships. There’s a fighting couple, a dreamily happy couple, a flirting couple — each vignette set to a song like “Strangers in the Night,” “One for My Baby,” or, twice, as sort of a double finale, “My Way.”

If not as exhausting (or sweat-filled) as “Upper Room,” this piece is certainly demanding on its dancers, with each duet full of complicated lifts and challenging partnering maneuvers. Delgado and Danny Ulbricht laid on the charm and verve in “That’s Life,” and Trenary, so sharp and effective in “Upper Room,” was equally impressive along with Benjamin Freemantle in a challenging duet to “One for My Baby.”

Tharp told The New York Times she chose “Upper Room,” a natural evening-closer, to instead open this show because it exemplified survival at a time when live performing arts like dance, not so long ago, were shut down with no assurance of when they’d return. And yes, the dancers at the end looked thrilled to have “survived” — but also energized, and exhilarated. As the crowd felt, too, when it jumped to its feet.