Defence lawyers for an army general facing sexual assault charges say his accuser’s testimony gave them plenty to challenge as they prepare to cross-examine her on Monday.

The army captain at the centre of the closely watched case testified on Friday that Brigadier General Jeffrey A Sinclair twice forced her to perform oral sex on him during their three-year, illicit affair. The woman said the assaults took place in Afghanistan in late 2011 as she grew increasingly despondent over their adulterous relationship.

Both times, she said, they got into arguments that ended with Sinclair unbuttoning his pants and forcing her head into his lap as she cried.

Sinclair, the 51-year-old former deputy commander of the storied 82nd Airborne Division, is believed to be the highest-ranking US military officer ever court-martialled on sexual assault charges. He could get life in prison if found guilty.

Sinclair pleaded guilty on Thursday to adultery and other charges that could send him to prison for up to 15 years. His lawyers are hoping the plea will limit some of the salacious evidence and reduce the case to his word against hers. Defence lawyers will probably ask the woman about some of the thousands of text messages she exchanged with the general, many of them sexually explicit.

Four months after she claims Sinclair sexually assaulted her, records show the captain traded a series of affectionate emails with the general. The captain signed off by saying she “owed” him, a code word defence lawyers say the paramours used as a substitute for “love”.

“I hope you are doing super great and feel the positive energy I’m trying to send you way!” she wrote to her alleged abuser. “You will know it’s working if you feel SUPER GREAT today!!! I will always owe you the most Sir!”

The trial is unfolding with the Pentagon under heavy pressure to confront what it has called an epidemic of rape and other sexual misconduct in the ranks. On Thursday, the Senate blocked efforts to pass a bill that would have stripped military commanders of authority to decide whether to prosecute rapes and other serious crimes. The bill would have given decisions to take serious crimes to courts martial to seasoned military trial lawyers, independent of the chain of command.

Sinclair’s accuser testified that the first assault took place in her office in Kandahar when she told him how she hated her life, hated working for him and believed the general was just using her for sex.

“He grabbed me by the back of the neck and pushed me down. I tried to pull back, and he put his other hand on my shoulder,” she said. “It felt disgusting. It felt like I had no control over my body.”

She said the second assault took place in his office days later after she asked him to move her to another job. She said that after that episode, they never had sex again.

Asked why she did not come forward and report what happened until March 2012, she said: “I knew if I said anything, it would be my word against his and no one would believe me. I had no way out.”

She also testified in detail about how the affair developed, and said that Sinclair once threatened to kill her and her family and “do it in a way no one would ever know” if she ever told anyone about their relationship. Still, the woman admitted she continued to have sex with him.

She was given immunity in exchange for her testimony. Adultery is a crime under military law.

In opening statements, prosecutor Lieutenant Colonel Robert Stelle painted Sinclair as a man who used his authority to intimidate and coerce a female officer nearly 20 years his junior into sex. Defence lawyers portrayed Sinclair as the victim of a desperately infatuated woman who became angry when she discovered that another woman had sent the general an email that closed with “I love you”.

The defence also contends Sinclair is the target of overzealous prosecutors under political pressure to make an example of him, despite weak evidence and an accuser with a history of telling lies.

The defence is expected to attack her credibility on Monday and says she lied about when she found a cellphone with text messages between her and the general. They also point out that when she first confided to a colleague about the affair in March 2012, she made no mention of being assaulted.

Defence attorney Ellen C Brotman began her opening statement by reading the jury composed of five two-star generals excerpts from the woman’s diary, written during the affair. Even after he was alleged to have threatened to kill her loved ones, she wrote of her desire for him, Brotman said, and the only fear she expressed was that he might still love his wife.

“I’m so in love with him,” Brotman read from the diary. “I do know that I love him incredibly … I love him almost unconditionally.”

Their relationship began in Iraq in 2009. He was her direct commander, a paratrooper revered for bravery under fire. She was a young intelligence officer and Arabic linguist, assigned to accompany him on trips to meet Iraqi leaders. She said that after months of conversation and flirtation, she went to his quarters late one night and they watched a television show. He asked her to put down her hair, which she typically wore in a bun while in uniform. She complied, and he began stroking her hair. They kissed that night, and the relationship soon evolved into frequent sexual liaisons.

“It was wrong,” she testified. “I knew it was wrong.”

She testified that he often spoke of his wife and children and that he described his spouse as an open-minded woman who understood he needed to have sex while overseas. She said she assumed Sinclair had told his wife about her.