Gwyneth Paltrow's ski crash trial got heated as an expert witness and the daughter of Paltrow accuser Terry Sanderson took the stand Thursday morning.

Paltrow is being sued for $300,000 by the retired optometrist, who claims the actress skied into him from behind and left him severely injured at Utah's Deer Valley Resort.

Gwyneth Paltrow exits the court in a three piece grey suit holding a blue notebook as a man wearing all black opens the pew door for her

Gwyneth Paltrow appeared in court Thursday as another expert witness took the stand along with one of plaintiff Terry Sanderson's daughters. (AP Photo/Jeff Swinger)

Sanderson's daughter, Polly Grasham, testified about the moment she realized something was "terribly wrong" with her father, about a year or year and a half after the ski collision.

"His processing speeds," she said, were slower and "the effort that it took, definitely when we were in person… I felt like, wow."

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"I almost expected drool to be coming out of his mouth because he was not engaged with anyone and had taken himself to a remote corner," Grasham added.

That's the moment she knew something was "terribly wrong."

Later in her testimony, she recalled an incident that occurred between her father and younger sister, Jenny, that left her feeling "terrified."

While helping clean out their grandparents' house, Jenny and Sanderson allegedly got into an argument. Grasham walked into the kitchen to find her dad yelling "f--- you" "over and over again" to Jenny.

"I've never felt scared of my dad," she told Sanderson's attorney. Bob Sykes. "I don’t think I ever received a spanking from my dad. I was terrified."

Gwyneth Paltrow enters the courtroom.

Paltrow arrives in court Wednesday. (Rick Bowmer)

During the third day of testimony, the jury also saw Paltrow's lawyer James Egan and the plaintiff's expert witness Dr. Alina Fong get into a heated exchange over her credibility.

Egan pushed to understand why Dr. Fong had not examined Sanderson's prior medical history when treating him during a neuropsychologist "boot camp" in 2017.

"Is it not possible that Mr. Sanderson had a concussion, his concussion symptoms resolved, and then he's continued to have other problems that you treated him for that were not concussion-related?" Egan asked. "Is that not possible?"

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"I mean, anything is possible, but it's not probable because I think you are completely ignoring the fact that he had these prior issues which makes him more fragile and significantly more at risk for these issues," the doctor responded.

Dr. Fong also accused Egan of "miscategorizing" her.

"I didn’t treat Terry any differently than any other patient that comes into my clinic," Dr. Fong said. "I take the patients as they come."

Gwyneth Paltrow leaves the courthouse

Paltrow leaves the courthouse Tuesday after the first day of the trial. (AP Photo/Alex Goodlett)

During day two of the trial, neuropsychologist Dr. Sam Goldstein testified that Sanderson, now 76, had an "acute change" in his behavior following the collision and has "become obsessed" with getting back to normal.

"He has become obsessed with trying to return himself to the level of functioning he perceived he had before this accident," Goldstein said on the stand.

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During cross-examination, Egan said, "He felt like he was abandoned on the slope and he kind of obsessed about that," to which Goldstein responded, "Correct."

Goldstein said Sanderson's problems were "more behavioral, emotional regulation." He added, "From his view, he is not the person he was."

Gwyneth Paltrow leaves the courthouse

Terry Sanderson claims actor turned lifestyle influencer Paltrow was cruising down the slopes so recklessly that they violently collided and he was severely injured. (AP Photo/Alex Goodlett)

Radiologist Wendell Gibby also discussed perceived changes in Sanderson's personality that occurred after the collision.

"Terry had been a very high-functioning, high-energy person," Gibby said. "Every day, he was doing lots of things. Meeting groups, wine tasting, skiing, volunteering.

"But, after his accident, he deteriorated abruptly, and many of the activities that he used to do, he stopped doing, like, for the most part… He normally could, you know, handle multiple projects at once, but he would have to sit there and focus very hard on one task. He would go to a Home Depot, for example, and forget why he was there."

Paltrow's lawyers are prepping for Paltrow to take the stand Friday. Paltrow's attorney said the jury will also hear from her husband Brad Falchuk and her kids, Moses, 16, and Apple, 18.

Terry Sanderson arrives at court.

Terry Sanderson looks on in the courtroom Tuesday. (Rick Bowmer)

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Gwyneth Paltrow looks on as she sits in the courtrooms

Paltrow has claimed Sanderson hit her and countersued for $1, plus attorneys' fees. (AP Photo/Rick Bowmer, Pool)

Paltrow and Sanderson are in litigation over a 2016 ski collision that left the retired optometrist severely injured, according to a 2019 lawsuit obtained by Fox News Digital. Much of the testimony so far comes from doctors speaking on Sanderson's medical condition prior to and following the collision.

Sanderson accused the Goop founder of skiing off after the accident, which left him with a "permanent traumatic brain injury, four broken ribs, pain, suffering, loss of enjoyment of life," along with emotional distress and disfigurement, according to the lawsuit.

Sanderson originally sued the actress for $3.1 million and claimed he was a victim of a hit-and-run. A judge dismissed the claim, and Deer Valley Resort and an instructor were removed from the lawsuit.

Paltrow has maintained that Sanderson actually skied into her and claims she stuck around until given the OK to leave by a Deer Valley Resort ski instructor. The 50-year-old actress also said in her countersuit that Sanderson previously admitted he didn't have a clear memory of the accident.

The actress is seeking attorneys' fees and $1.

View of Deer Valley Resort

The ski slopes at Deer Valley Resort in Utah. (Getty Images)

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Fox News Digital's Tracy Wright contributed to this report.