Attorney reveals his client killed visiting Chinese scholar Yingying Zhang

In a shocking courtroom revelation, the attorney for Brendt Christensen, a former teaching assistant at the University of Illinois, admitted that his own client had kidnapped and killed a young Chinese visiting scholar.

“Brendt Christensen is responsible for the death of Yingying Zhang," public defender George Taseff said during his opening statement in Christensen’s trial. Christensen had pleaded not guilty to the charges.

Yingying Zhang, 26, was last seen entering a black Saturn Astra on June 9, 2017 -- only a few yards away from a bus stop outside the university's PBS radio and television station. Within weeks, investigators tracked that car back to Christensen.

"Brendt Christensen killed Yingying Zhang,” Taseff went on. "Nothing that we say or do will be meant to sidestep or deny that."

If convicted, Christensen, 29, could face the death penalty.

“He is on trial for his life in this case,” Taseff told the jury.

Taseff spoke shortly after the prosecution's opening statement. The government confirmed for the first time that there is DNA evidence linking Christensen to the crime.

"He kidnapped her, he raped her, he assaulted her, he covered up his crime,” Assistant U.S. Attorney Eugene Miller repeated a total of three times during his opening statement.

In another startling disclosure, Miller said that Christensen allegedly told a wiretapped informant that Yingying was his 13th victim and that he'd been killing since he was 19 years old. Prosecutors say investigators have not linked Christensen to any other cases at this time.

Miller said the informant, Christensen’s then-girlfriend, recorded multiple conversations between herself and the defendant. Prosecutors described, in great detail, Christensen’s alleged admission of what he had done to Zhang on these recordings.

“[Christensen] said she fought for her life,” Miller told the jury. He also said Christensen told the informant he raped and beat Yingying before assaulting her with a baseball bat and eventually decapitating her.

Blood stains on the baseball bat investigators found in Christensen’s apartment, along with multiple other stains found on his mattress and carpet, matched Yingying's DNA, according to prosecutors.

According to Miller, Christensen did not reveal what he did with Yingying’s body in the undercover recording, but said “her family would leave empty handed” and that the last serial killer at his level was Ted Bundy.

Taseff said the defense took issues with "the way the government says the events occurred." Taseff alleges that Christensen was lying when he told the informant there were 12 other victims.

Taseff painted Christensen as a troubled graduate student at the University of Illinois who was dealing with substance abuse issues, a failing marriage and a plummeting academic record. Taseff said all of these issues created the “perfect storm” on June 9, 2017, when Christensen “did the unthinkable.”

Zhang’s father, brother and boyfriend of eight years sat in the front row of the courtroom. Her mother, Lifeng Ye, watched from another courtroom on a closed circuit stream. Zhang’s boyfriend, Xiaolin Hou, was called to testify shortly after opening statements.

“We planned to get married that October,” Hou told prosecutors.

Opening arguments ended by 10:30 a.m. Wednesday. More witnesses are expected to testify.

ABC News' Elle Luan contributed to this report.