Tristan and Isolde: Deborah Voigt as Isolde. (A Bofill Gran Teatre del Liceu/ Courtesy of Kennedy Center)

This weekend, the Washington National Opera delivered a bombshell. Deborah Voigt, the star soprano scheduled to sing Isolde in Wagner’s monumental “Tristan und Isolde” in the company’s season-opening production, starting next Sunday, was withdrawing from the role.

Singing in her stead are Irene Theorin, the Swedish soprano already familiar to WNO audiences from “Siegfried” and other roles, and, for the final performance on Sept. 27, the British soprano Alwyn Mellor, who scored a success as Brünnhilde in Seattle this summer.

To some, Voigt’s action sounded like diva temperament at its worst. But as Voigt, 53, revealed in a frank conversation in a Foggy Bottom restaurant Sunday, the day after the announcement, it was a mutual decision arrived at in her dressing room in a conversation with Francesca Zambello, WNO’s artistic director, who has worked with Voigt for nearly 30 years and is by now a close personal friend.

No great disaster led to this decision. There was simply a feeling, in rehearsals, that some of the toughest passages of this notoriously tough role — like the famous intoxicating climax of Act II — weren’t coming as well as she wanted. After finishing up a last run as Brünnhilde in the Metropolitan Opera’s new “Ring” cycle, another punishing role, Voigt had taken time off this summer, some of it inadvertently when a stomach ailment forced her to cancel another new Wagner role, Ortrud in “Lohengrin,” at short notice. Now, she was having trouble getting her voice back to where she wanted it.

“I probably would have soldiered on,” she says, and done the performances. But after the piano dress rehearsal, Zambello came into her dressing room and asked to talk. She asked, Voigt says, how Voigt was feeling. She mentioned that she needed the opening, the first of her administration, to be really, really good. And then “I burst into tears,” Voigt says, “and we had a good cry.” When she had recovered, she asked Zambello whom she had in mind to replace her.

“There’s a line in the Big Book of [Alcoholics Anonymous], of which I am a member,” she says, “that ‘the grace of God can do for us what we cannot do for ourselves.’ ” That, she says, is how she feels about this cancellation. One of her main emotions is “relief.”

“I am so tired,” she says, “of competing with myself” — with her younger self, that is, preserved on records and in the memories of her many fans. “I’ve done everything I wanted. It’s time to let her [in this case, Theorin] do it. I did the same thing to Jessye Norman when I was young. It’s a natural cycle.”

Isolde is one of the biggest roles in the repertoire; some (myself included) have always questioned whether it and Brunnhilde are too large for Voigt’s shimmering, powerful soprano. Others point to the weight-loss surgery Voigt underwent in 2004 as a reason her voice seems to have changed (Voigt says she wishes she had $50 for every time she’s heard that her voice was more golden before the surgery, and more silvery afterward). “Physiologically, it shouldn’t make a difference,” she says, of the surgery’s effects on her singing. “But I will say it took me longer to learn to sing again afterwards than I had expected.” However, she ascribes any changes that have taken place much more to “the changes that come as a dramatic voice matures” and as a woman gets older.

Can Voigt still sing Isolde? On Sunday, she mused about trying out character roles usually taken by older sopranos (like the Kostelnicka in Janacek’s “Jenufa”); talked about exploring acting roles, and a cabaret show she is hoping to get off the ground; and stressed the importance of moving into the next phase of her career with grace. At the same time, she knows she still could physically have sung the role; some people would have applauded her. Listening to a clip of her last outing as Isolde, in Spain in 2010, which I posted on the Classical Beat blog this weekend, she said, “It’s not like I think, ‘Oh, I was singing so much better then than I am now.’ ”

We pretend that the world of art is black and white; that a singer can or cannot sing a role; that someone is either fired or quits. But the truth is far harder to pin down. Voigt’s career is by no means over; she is performing actively in recital and in opera for the rest of the season. She will return to Washington several times this season as the Domingo-Cafritz program’s first artist-in-residence, and she is reportedly returning to WNO in a future production. It’s true that there are no Isoldes or Brunnhildes coming up on her calendar. But while this cancellation is “a chance to take stock of everything, really,” she isn’t ruling anything out.

“Maybe someone else will cancel an Isolde,” she said, laughing, “and I’ll get called in as a replacement.”