Paltrow's lawyer Steven Owens told the Park City court on Tuesday that Sanderson is "obsessed" with the lawsuit, and that the case was a "meritless claim of false allegation."
"Really kind of an offensive one. That she somehow left him an unconscious man and bolted? I can tell you, we believe it to be utter BS," said Owens.
The alleged collision occurred in February 2016 at Deer Valley, a glamorous ski resort above the swanky larger resort of Park City in the Rocky Mountains.
Deer Valley hosted Winter Olympics skiing events in 2002, while Park City is home to the annual Sundance film festival.
Paltrow, wearing a white turtleneck sweater, sat silently beside Owens as proceedings began Tuesday. She is expected to later take the stand in her own defense.
Paltrow's husband Brad Falchuk and children Moses and Apple are due to address the court, Owens said.
Sanderson says Paltrow skied into him from behind and then vanished, leaving him unconscious, lying in the snow.
Lawrence D. Buhler, representing Sanderson, said in his opening statement that Paltrow had been skiing in a "dangerous" and "reckless" manner, and had caused his client "four broken ribs and permanent brain damage."
Paltrow says Sanderson skied into her back, and is countersuing for a nominal $1 plus legal expenses.
Her lawyer Owens said "Gwyneth is a conservative skier" who was "not going fast," and said she had initially feared she was "being assaulted" when Sanderson appeared directly behind her.
"It rattled her and physically hurt her," he said.
Owens also said Sanderson is "blind in one eye" and suffers from "decreasing vision" in the other -- suggesting poor eyesight was a reason for the crash.
In addition to her Oscar-winning acting career, Paltrow has forged a second career marketing wellness products on her Goop website.
The online blog and store promotes healthy eating and stress-free living, as well as a range of unorthodox products -- from DIY coffee enema kits to a line of scented goods made with cedarwood, bergamot, rose and ambrette, and named "This Smells like My Vagina."