The theater world was shocked and saddened yesterday to learn of Pulitzer-winning playwright Wendy Wasserstein's battle with a life-threatening disease.

For weeks, there had been rumors that she was ill, but the nature and seriousness of her illness only became apparent yesterday as word raced around Broadway that Wasserstein, 55, was on life support.

During rehearsals of her latest play, "Third," which is currently running at Lincoln Center Theater, Wasserstein often seemed fatigued, people involved in the show told The Post.

At one point, Andre Bishop, the head of Lincoln Center Theater, feared she would not be able to complete some rewrites of scenes in time for the opening, sources said. But she persevered, and "Third" opened to generally positive reviews, with several critics saying it was her best play in years.

It's also proved popular with Lincoln Center audiences and has been extended until Dec. 18.

"Third" stars Dianne Wiest as a left-wing college professor named Laurie Jameson who has it in for a white male student largely because he's a Republican.

She accuses him of plagiarizing a paper on "King Lear" and hauls him before the Stalinist-like Committee on Academic Standards.

Wasserstein, who once called herself a "typical New York playwright liberal," does not write black-and-white characters, however, and many critics were struck by the fact that in "Third" she seemed to be questioning the growing intolerance of those on the left.

The play also deals, quite poignantly and, sadly, ironically, with illness and death: Laurie's best friend is dying of a

cancer, while her father is suffering from Alzheimer's.

Laurie Jameson is one in a long line of independent-minded (and usually single) feminists who have populated Wasserstein's tender and witty plays.

Her most famous creation is probably Heidi Holland, the title character of "The Heidi Chronicles," which won the Pulitzer Prize and the Tony Award in 1989.

Holland is a feminist art history teacher struggling with the realization that her professional accomplishments cannot fill the void of a lonely personal life.

Loneliness also was one of the themes of Wasserstein's 1993 Broadway hit, "The Sisters Rosensweig," which deals with a successful female investment banker who has come to believe that her intelligence and power have driven men away.

Wasserstein's other plays include "Uncommon Women," about a group of female friends who have just graduated from a Seven Sisters school, and "Isn't It Romantic?" a popular comedy about a young woman living on her own for the first time in New York City.

Like many of her characters, Wasserstein has never married. In 1999, she gave birth to a daughter and has written and lectured extensively about her life as a single parent.

A beloved figure in the theater world, Wasserstein also helped create a much-praised educational outreach program for the Theater Development Fund, which brings Broadway's top writers, directors and performers to classrooms throughout the city.

Wasserstein's wide circle of friends includes director James Lapine, composer William Finn, New York Times columnist Frank Rich and playwright Terrence McNally.