A top aide and ex-boyfriend of fallen Theranos founder Elizabeth Holmes went on trial Tuesday in California, with prosecution and defense clashing on allegations Ramesh "Sunny" Balwani helped perpetrate massive fraud at the startup.

Holmes was convicted in January of cheating investors in the blood testing company once hailed as revolutionary and valued in the billions of dollars, but which collapsed under claims its technology did not work as promised.

US prosecutor Robert Leach told jurors in a federal courthouse in San Jose, the heart of Silicon Valley, that Balwani piloted the firm alongside Holmes.

"The evidence will show that they ran the company together. You will see how they were partners in everything, including their crime," Leach said in his opening arguments.

But 56-year-old Balwani's attorney Stephen Cazares said his client never committed fraud, and was convinced of Theranos's potential.

"He did not start Theranos, he did not control Theranos," Cazares said. "Sunny believed in Theranos, he believed in its technology and he believed in its mission."

Balwani, nearly two decades Holmes's senior, was brought in to help steer the company she had founded in 2003 at age 19. He has pleaded not guilty.

Former Theranos COO Ramesh "Sunny" Balwani arrives in court on March 16 for proceedings in his fraud trial
Former Theranos COO Ramesh "Sunny" Balwani arrives in court on March 16 for proceedings in his fraud trial GETTY IMAGES NORTH AMERICA via AFP / JUSTIN SULLIVAN

Holmes, now 38, would go on to promise self-service testing machines that could run an analytical gamut cheaply and on just a few drops of blood -- a pledge shattered under fraud allegations.

"What you are going to learn in this trial is that the investor money they obtained was used exactly as the investors intended," Cazares added. "And you are going to learn that Sunny was one of those investors."

Prosecutors alleged Holmes -- and Balwani -- were aware the technology did not work as advertised, but continued to promote it as revolutionary to patients and the investors who poured money into the company.

"The evidence will show that the defendants knew that the rosy falsehoods that they were telling investors were contrary to the reality within their lives," Leach argued, citing misrepresentations of the nature of the company's contracts with Walgreens and Safeway.

As Theranos soared, it attracted luminaries such as Rupert Murdoch and Henry Kissinger, but a series of reports casting doubt on the firm's claims from Murdoch's own Wall Street Journal set the company's collapse in motion.

The paper's series of stories raising questions about the much-hyped technology heralded the beginning of the end for Theranos.

Holmes was convicted in January after a high-profile trial, and now faces the possibility of a prison term when she is sentenced at a hearing set for September.

During her trial, Holmes alleged that Balwani was emotionally and physically abusive during their romantic relationship -- allegations he has denied.