Nicollette Sheridan told a court she was stunned after being struck by the creator of Desperate Housewives on the set of the ABC show.
The actress re-enacted the hard blow on the head she claims series creator and executive producer Marc Cherry inflicted during a dispute over a scene in September 2008.
Her character was killed off the show months later and her lawyers claim Cherry made that decision after he was cleared of wrongdoing by network executives.
Sheridan was the first witness called at the Los Angeles trial. She split the day between testifying about the early years on Desperate Housewives, and the dispute with Cherry and its aftermath. The 48-year-old actress is seeking more than six million dollars (£3.7m) for wrongful termination and battery. Cherry has denied wrongdoing.
The case centres on whether Sheridan's complaints about Cherry's slap - described by her lawyer as a hard whack, and a defence lawyer as a tap meant to give artistic direction - led to her character Edie Britt's violent demise and the loss of her 175,000 dollar (£110,000)-an-episode job.
Sheridan's lawyer Mark Baute claims her character was killed off only after ABC cleared Cherry of wrongdoing.
Adam Levin, who represents Cherry and ABC, maintains the decision to kill off Sheridan's character came months before the on-set dispute and was made after consulting the highest ranks of the network's management.
He noted on Wednesday that Sheridan did not suffer any physical injuries and denied Mr Baute's accusations that Cherry or other show employees tried to cover up the row between the producer and Sheridan.
The show's stars Eva Longoria, Teri Hatcher, Felicity Huffman and Marcia Cross all are listed as potential witnesses.
Sheridan's character died in the show's fifth season after being nearly strangled, involved in a car crash and finally, electrocuted.