WAUKESHA, Wis. — The young woman marched confidently to the witness stand with her head high, her long, black hair bouncing with each step. Her voice, though difficult to hear, didn't tremble as she was sworn in. As the 5-foot-2-inch woman took her seat, she cast a glance at the defendant — former Green Bay Packer Mark Chmura.

And the story of what happened the night the 31-year-old NFL star allegedly sexually assaulted his former babysitter began to unfold.

The witness, 18-year-old Kim, is the friend of Chmura's accuser, and she claims to have been sexually assaulted the same night by Chmura's friend Robert Gessert, 43, the host of the after-prom party at which a group of teens and the two men drank heavily into the early hours of the morning. She also claims to have seen the alleged victim, 17-year-old Allison, emerging from the bathroom with Chmura and to have heard Allison's account of the forced sexual intercourse she claims took place behind the closed door.

Chmura now faces charges of sexual assault and child enticement and could face 40 years in prison if convicted.

On this first day of testimony, the jury — who were selected in Janesville, 90 minutes from here, and are sequestered at a local hotel for the duration— District Attorney Paul Bucher asked his first witness about her level of intoxication and that of the other teens in attendance, as well as Chmura and Robert Gessert, the owner of the house.

Kim had been to two parties earlier that evening, she said. She didn't have a date to the prom and gathered instead with several other girls who opted not to go to the dance. They embarked on a long night of high-octane cocktails, chased with beer and nightcapped with a strong concoction of whisky and Mountain Dew. She had three to four drinks at the first party, she testified, before heading off to another party. She said she didn't drink at the second party and arrived at the Gessert household shortly after midnight. Gessert's daughter, who went to high school with the alleged victim and Kim, was throwing the party.

"It was very laid back," she said, describing the scene in the Gessert's basement before the arrival of the defendant. "People were just hanging out on couches and talking." The beverage of choice for the youths was vodka. There was a pitcher set out on the downstairs card table, a fusion of the liquor and lemonade, she said.

And then Chmura showed up.

"He was drunk," Kim testified, describing the ex-Green Bay Packer's slurred speech and swaying body movements. She said the defendant posed for pictures and knocked back a few beverages with some of the high school football players in attendance. He then took the horseplay up a level by calling Packers teammate Frank Winters from his cell phone at about 3:30 a.m.

Afterward, Kim witnessed a drinking game in the basement between the alleged victim, Chmura, Robert Gessert and high school football player Michael Kleber, who seemed to be enamored of Chmura, she said. The boy, then 16, hung on his every move and word. At one point, Chmura smashed a glass filled with vodka on a ping-pong table with a paddle. Kleber, trying to impress the pro athlete, Kim said, licked the drink off the ping-pong table, undeterred by the glass shards.

The game went on for about half an hour. "Allison was losing miserably," Kim recalled.

Her friend started slurring her speech and staggering around the basement. When the game was over, Kim said that Gessert announced, "It's hot tub time." A crew of youths flocked to the hot tub. Kim openly told them that she didn't have a suit. She says Gessert told her that "brown underwear was fine," so she undressed and got into the tub in her underwear.

But then the party went sour. Kim began to feel sick and then vomited over the side of the hot tub. Not long after, she said, Gessert sexually assaulted her: He slid a finger inside the front of her panties and stuck the finger in her vagina. Her claim is not the subject of this proceeding. Gessert is scheduled to go on trial in March for the alleged assault.

As the witness described the incident, she began sobbing, mascara carried down her cheeks by her tears. She testified that she ran out of the tub, struggled to get inside the house and then ran up the stairs to Gessert's daughter's room. There, she told several of her friends what had just happened.

During her testimony, Kim also discussed Allison's alleged dislike for Chmura before that night. Allison knew Chmura and his wife, Lynda, from babysitting their two boys. Kim testified that on several occasions, Allison said Chmura called her "jail bait." Allison also had told her that Chmura "had a baby with a babysitter," Kim testified.

But during cross-examination, defense lawyer Gerald Boyle attempted to discredit Kim by questioning her judgment at the time of the alleged crime. The grandfatherly attorney tried to bring out a different side of Kim, Allison and their friends. Boyle got the young woman to admit on the stand that she and about five of her friends, including Allison, call themselves the "SBs" — the "Sexy Bitches."

When asked about her inebriation on a scale of one to 10, Kim said she was an eight. She also said Chmura, Allison and homeowner Gessert were eights. But she added that Kleber, whose testimony the defense hinges on, was a 10.

"He couldn't talk," she said.

Boyle took the opportunity to try to support Kleber's statements. Boyle asked Kim if she had witnessed a situation mapped out in Kleber's initial statement to police. Kleber wrote in his statement that after Allison emerged from the bathroom, he asked her what she had just done with Chmura. According to his statement, "She said: 'Nothing. I swear on my life. I swear on my parents' life. I swear on the holy Bible. I didn't do a thing.'"

Kim said she had heard Kleber ask Allison this question, but denied the alleged answer. "All she said was 'nothing,'" Kim testified.

Her story lost its momentum at times during cross-examination because she couldn't remember certain details. Many of her answers were "I don't recall" or "I don't know," and the lawyer's questions frequently prompted her to roll eyes.

Kim also described for the defense lawyer the moment she witnessed Allison emerging from the bathroom. Kim testified that she saw the bathroom door swing open, and Chmura and Allison stood on the threshold together — fully clothed.

The next day, she said, she and Allison discussed what had happened. While Allison related the alleged sexual assault to Kim, she constantly reiterated that the two had "nonconsensual sex."

"She said 'nonconsensual,' in those words," Kim testified.

Kim's testimony lasted more than four hours. During this time, Chmura sat in a seeming stupor, staring blankly ahead. He studied Kim's face from time to time, but when she described him luring Allison into the bathroom, Chmura hardly seemed to blink.

The prosecution will continue to mount its case Friday morning and may continue through the weekend. The alleged victim, Allison, is expected to testify, as are Kleber and the nurse that treated Allison at the sexual assault trauma center.