The 54-year-old was reported to be in a stable condition after reconstructive surgery.
Police officer G Sparjan Kumar said the woman fled her home after the attack on Friday night and called police.
When he again visited her home on Friday night and tried to force himself on her, she got hold of a knife and attacked him, Mr Kumar said.
The New Delhi Television news channel said the woman's family knew Theerthapada, who used to visit their home to cure her bed-ridden father.
She told police he would rape her whenever he had an opportunity.
Pinarayi Vijayan, the state's chief minister, told reporters it was brave of the woman to take such action.
"It's a courageous and strong act by the woman," he said.
Violent crimes against women have been on the rise in India despite tough laws enacted by the government.
Last week, an Indian woman was gang-raped then brutally murdered by men who smashed her skull with bricks in the northern state of Haryana.
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