Defense seeks to strike testimony at Clemens' trial WASHINGTON (Reuters) - Attorneys for ex-pitching ace Roger Clemens sought on Monday to strike a key witness' wavering testimony that Clemens once said he used performance-enhancing drugs, a critical element of the perjury case against him. Andy Pettitte, a former New York Yankees teammate of Clemens, agreed last week under defense cross-examination that he was "50-50" about whether Clemens had told him in 1999 or 2000 he had used human growth hormone. Pettitte's testimony was considered key to the prosecution's case that Clemens had lied to Congress about using performance-enhancing drugs during his 24-year career. "The court should not allow the jury to consider an alleged 'admission' that has all the weight of a coin flip," the defense said in a court filing on Monday. Clemens' attorneys, headed by Houston lawyer Rusty Hardin, asked that Pettitte's testimony about the conversation be struck from the trial record. They said in part that it had failed to reach the "preponderance" standard for evidence of more than 50 percent. They asked that the prosecution's reference to the conversation in its opening statement be struck as well. Clemens is being tried for a second time on federal charges of lying to the House of Representatives' Committee on Oversight and Government Reform in 2008 about whether he used performance-enhancing drugs. Clemens, a seven-time winner of the Cy Young Award, baseball's highest annual honor for a pitcher, is among the game's biggest names linked to alleged drug use. The trial on Monday centered on testimony by former Internal Revenue Service agent Jeff Novitsky and Federal Bureau of Investigation agent John Longmire about the handling of evidence supplied by Brian McNamee, Clemens' and Pettitte's former personal trainer. The evidence included vials, needles, tissues, syringes and other material, some of it stuffed inside a Miller Lite beer can, that McNamee turned over in January 2008. Prosecutors said last week that McNamee could testify on Tuesday. The prosecution is expected to use him to show that the trainer injected Clemens with anabolic steroids in 1998, 2000 and 2001 and with human growth hormone in 2001. Clemens' attorneys have called McNamee a liar. Reporting by Ian Simpson; Editing by Cynthia Osterman