The show goes on for cancer-stricken singer

Soul singer Sharon Jones is the focus of a new documentary that captures the stark difference between life on and off stage as she battles cancer.

NEW YORK — Preparing to go on stage for the first time in months after intensive rounds of chemotherapy, an atypically nervous Sharon Jones sat backstage at New York’s Beacon Theatre, clutching a cup and shaking.

“Then they announce her — ‘Miss Sharon Jones!’ — and she goes like a prizefighter onto the stage,” recalls Barbara Kopple, the Oscar winning filmmaker. “And she just kills it.”

It’s one of the many moments in Kopple’s documentary “Miss Sharon Jones!” that captures the stark difference between life on and off stage for the dynamic soul singer most often compared to James Brown.

In 2013, Jones was diagnosed with stage-two pancreatic cancer. The film documents her transformation into cancer patient and, ultimately, back into a full-throated force.

Yet what might have culminated in triumph has instead been complicated by the cancer’s reoccurrence, which Jones announced at the film’s Toronto Film Festival premiere last fall. She has continued to perform, but she’s currently on pain medication and recently underwent a blood transfusion.

On a recent off-day during her tour with the Dap-Kings opening for Hall & Oates, an exhausted Jones laid her head on the table of an Upper East Side bar.

“I had to take the chemo to get me prepared for the road,” Jones says. “I basically have to worry about the shows and getting up there and having the energy and the strength to get through those. So anytime downtime I have, I’m down.”

The documentary has, the 60-year-old singer says, turned into a kind of motivation for her second round with cancer: visual proof that she got through this once before, and can do it again.

Kopple, the filmmaker of groundbreaking documentaries like “Harlan County, USA” and “American Dream,” didn’t meet Jones until she began filming. Their first day together was when Jones had her head shaved for chemo.

“I think it gave her a real sense of trust and, on my behalf, a real sense of love for this woman who just has incredible strength and perseverance,” Kopple said.

Those are traits, along with a soulful wail that sounds straight out of Motown, that brought Jones fame in the first place. It didn’t come until the South Carolina native was 40 years old, following years of working blue-collar jobs in New York, even as a corrections officer at Riker’s. She was eventually brought in as a frontwoman for Daptone Records. Some half-a-dozen records have followed, which staked an early claim to soul music revivals (the Dap-Kings backed Amy Winehouse) and created some classic funk workouts and R&B ballads like “100 Days, 100 Nights.”