When the double murder trial began, the Spanish-speaking Salvadorean maid was billed by Simpson's lawyers as a crucial witness. Here was someone who claimed to have seen his Bronco car outside his mansion at the time when prosecutors allege he was murdering his ex-wife and Ronald Goldman, several miles away. What better proof of his innocence?
But, as her story has unfolded, her credibility has been dealt blow after blow. Under cross-examination, she has repeatedly claimed to be unable to remember things, replying "no me recuerdo" no fewer than 89 times on Thursday - an unfortunate trait in a witness whose memory is so crucial to her claims.
Some of her apparent memory lapses have been puzzling. When the prosecutor, Christopher Darden, asked her if she told a friend she would receive $5,000 (£3,200) if she, too, would say she saw the Bronco outside Simpson's house, Ms Lopez said she could not recall this. As legal commentators observed, it was not the kind of conversation that slips the mind.
The maid has also been caught lying. She falsely claimed to have made a reservation for a flight home to El Salvador, but later admitted this was untrue. Mr Darden also extracted an admission that she had applied for unemployment benefit (despite her planned departure), and not, as she had claimed, merely filled out forms.
More critically still, she no longer appears specific about timing. Although she has stuck by her claim that she saw Simpson's vehicle on the night of the murders (12 June last year) she told the Los Angeles Superior Court that this was "some time after 10" - not between 10.15pm and 10.20pm, as the defence claimed. She also seemed to concede that the precise times had been suggested by a defence investigator during an interview.
Nor is her image improved by prosecution suggestions - denied by the defence - that she has been coached, and even bribed. She has admitted accepting new clothes as gifts, and a$600 loan from defence lawyers.
Johnnie Cochran, Simpson's lead attorney, has avoided saying whether Ms Lopez's erratic testimony will be played to the jury - it was pre-recorded for later showing because of fears she would leave the country.
But some lawyers now think it unlikely. "Rosa Lopez was an unmitigated disaster for the defence," said Roy Black, an attorney-commentator, "What good is an alibi witness who can't remember anything? ... I predict that we will never see this videotape during the trial." Californian trial- watchers now call her "credibility impaired". But yesterday's New York Post summed it up best: "Bye bye alibi".