"Physically I feel great!" the actress, 74, who has been battling a form of terminal brain cancer, told PEOPLE at Monday's show, where she earned a standing ovation for her 21-point foxtrot with Tristan MacManus.
At this point, she's just dealing with minor aches and pains.
"It's a knee that I strained during early rehearsals. Just age," she says. "Nothing very bad, but I've been getting therapy for it."
Harper has shown remarkable improvement in her condition on recent brain scans, coming "close to a remission," according to her doctor – which has buoyed her emotionally.
"I think about cancer less and less all the time," she says now. "I'm doing very well, and feeling well. I try to live every day, like we all should, baby. All of us. No one knows what's ahead."
With reporting by MICHELE STUEVEN