The founder of Theranos, the failed blood testing start-up, asked for a new trial after a surprise visit from a key witness to her house.

When Elizabeth Holmes, the founder of the failed blood-testing start-up Theranos, was convicted of fraud in January, the verdict represented the end of a drawn-out saga.

But in the ensuing months, as Ms. Holmes awaited her sentence, the drama around her case has only escalated.

First Ms. Holmes’s co-conspirator, who was the former chief operating officer of Theranos, was convicted of fraud in July. Then Ms. Holmes asked the judge to overturn her conviction based on a lack of evidence and submitted a flurry of requests for a new trial based on new evidence. At recent hearings over the case, Ms. Holmes has appeared visibly pregnant with her second child. And in August, a key witness did something highly unusual in a criminal case: He showed up at her house.

That incident became the basis of Ms. Holmes’s latest attempt to reverse her fortunes. On Monday, the 38-year-old, her parents and partner, lawyers and a scrum of media gathered in a courtroom in San Jose, Calif., for a hearing that could open the door to her getting a new trial. The visit by the key witness, Ms. Holmes’s lawyers argued, raised questions about his credibility and the fairness of the trial.

The move is a long shot, experts said.

“It is a near-certainty that the judge will deny Elizabeth Holmes a new trial” on the basis of the witness’s visit to her house, said Amanda Kramer, a former federal prosecutor who is a partner at the law firm Covington & Burling. The judge likely allowed the hearing to prevent Ms. Holmes from using the incident in her inevitable appeal, she added.

But little about Ms. Holmes’s case, which came to symbolize the pitfalls of Silicon Valley’s hype-driven start-up culture, has been typical. Ms. Holmes and her partner, Billy Evans, declined to comment on the case or whether they are expecting.

At issue is an Aug. 8 visit from Dr. Adam Rosendorff, who played a key role in Theranos’s rise as its lab director. He later became a whistle-blower who helped expose the company’s fraud. Theranos had told patients and investors that its revolutionary technology could accurately perform thousands of blood tests with a single drop of blood when it could not.

During Ms. Holmes’s trial last year, where she faced nearly a dozen counts of misleading patients and investors, Dr. Rosendorff endured six bruising days of testimony, the longest of any witness. After, jurors said they found his testimony among the most credible in the trial.

Then in August, Dr. Rosendorff visited Theranos’s former office in Palo Alto, Calif., as well as the first Walgreens store the company had worked with. Both, he found, were gone.

As a result, he “suddenly felt that a conversation with the defendant was the missing piece” to moving on with his life, his lawyers said in a filing. Dr. Rosendorff drove to Ms. Holmes’s residence in nearby Woodside, Calif. Her partner, Mr. Evans, answered, and told him to leave.

From there, the accounts differ. Ms. Holmes’s camp said Dr. Rosendorff expressed guilt over his role in the situation and said that government prosecutors had “made things sound worse than they were.” Ms. Holmes argued that the incident called Dr. Rosendorff’s testimony and the government’s entire case into question, which meant she deserved a new trial.

On Monday, Dr. Rosendorff returned to the stand. Judge Edward Davila, who oversaw Ms. Holmes’s trial, asked whether Dr. Rosendorff’s testimony at the trial was truthful and whether the government had faithfully represented the facts. He testified affirmatively.

Then Lance Wade, Ms. Holmes’s lawyer, grilled him. Why did Dr. Rosendorff want to visit Ms. Holmes? Had Dr. Rosendorff had a mental breakdown that impacted his testimony? Was the government trying to make everyone look bad? Was Dr. Rosendorff seeking to help Ms. Holmes?

Dr. Rosendorff responded by accusing Ms. Holmes’s lawyers of trying to paint him as a liar. He said he felt sympathy for Theranos employees who were impacted by the scandal — but not for Mr. Holmes and her co-conspirator, Ramesh Balwani. He added that he felt bad that Ms. Holmes’s children would grow up without a mother if she went to prison.

Ms. Holmes was convicted on four counts of fraud, with each carrying a maximum penalty of 20 years in prison.

Dr. Rosendorff testified that his contact with Ms. Holmes was motivated by a desire for healing.

“I don’t want to help Ms. Holmes,” he said. “She’s not somebody who can be helped. At this point she needs to help herself. She needs to pay her debt to society.”

Ms. Holmes stared at Dr. Rosendorff throughout his testimony, occasionally taking notes. As she left, arm in arm with Mr. Evans, she flashed a smile to reporters but did not respond to questions.

Outside the courtroom, Dr. Rosendorff ran away from a group of news cameras. A lawyer for Dr. Rosendorff declined to comment.

Judge Davila said he had received the answers to his questions regarding the incident. He will decide whether Ms. Holmes deserves a new trial in the coming weeks.

Ms. Holmes is scheduled to be sentenced on Nov. 18. She is expected to appeal.

Mr. Balwani, who was convicted of a dozen counts of fraud for Theranos, is set to be sentenced on Nov. 15. He tried to piggyback on the visit from Dr. Rosendorff to Ms. Holmes as a reason for his own new trial. The motion was denied.