The former world number nine was up against Ekaterine Gorgodze, of Georgia, at the Koser Jewelers Tennis Challenge in Pennsylvania, when she shocked the crowds with an unsettlingly lacklustre performance.
Vandeweghe was sharply criticised for refusing to chase after balls during a warm-up with her opponent, even switching to her non-dominant hand for forehands.
One critic accused her of being “petulant”, suggesting she was being stubborn and “refusing to do a proper warmup”, while a match commentator said he had “never” seen such behaviour “in my life”.
Following the public backlash, Vandeweghe took to Twitter to explain her actions.
The 29-year-old explained that she’d been suffering from a “severe sinus infection” as well as heat exhaustion.
“By the end of the second set my entire body was cramping intensely and I had extreme nausea. I struggled to finish,” she wrote in a lengthy statement.
“After the (heat) suspension was lifted my opponent requested a warm-up; I informed the umpire that I preferred to skip the warm-up, citing my exhaustion and general illness in an attempt to conserve energy for what would be a grueling third set under the conditions.”
She added that she had wanted to “conserve as much energy as possible during the warm-up in preparation to compete with all of my ability in the third set,” and so had recommended that Gorgodze warm up with her coach, but her rival “declined”, forcing her back onto the court.