Andy Pettitte leaves the U.S. District Court after testifying in...

Andy Pettitte leaves the U.S. District Court after testifying in the perjury and obstruction trial of former teammate Roger Clemens in Washington, D.C. (May 1, 2012) Credit: Getty

WASHINGTON -- Yankee pitcher Andy Pettitte threw prosecutors a curve Wednesday when he agreed there's a 50-50 chance that he misunderstood Roger Clemens in a decade-old conversation in which he says Clemens admitted taking human growth hormone.

The potential blow to the government's perjury case against Clemens came a day after Pettitte, a key prosecution witness, described for jurors that conversation as evidence that Clemens had used performance-enhancing drugs.

Citing that uncertainty, Clemens lawyer Michael Attanasio moved after Pettitte left the witness stand to strike Pettitte's testimony about the conversation from the record.

U.S. District Judge Reggie Walton is expected to take up the issue next week.

As the trial resumed Wednesday, Attanasio asked Pettitte whether he believed "in your heart and mind" that he might have misunderstood what Clemens said in that passing conversation during an offseason workout. "Could have," Pettitte said.

Attanasio then asked Pettitte whether it would be fair to characterize the chance that he misunderstood Clemens as 50-50. "I'd say that's fair," Pettitte said.

On Tuesday, under questioning by Assistant U.S. Attorney Steven Durham, Pettitte recalled the Clemens conversation and said, "Roger had mentioned to me that he had taken HGH and that it could help with recovery."

After Pettitte was excused as a witness Wednesday, Walton asked Durham why he didn't directly ask Pettitte about his suddenly shaky memory after the defense questioning ended.

"What we have now based on testimony by Mr. Pettitte is that he's not sure," Walton said. "His testimony now before the jury is, 'I don't know.' "

During the discussion with Walton, Durham said Pettitte already had said it was his recollection and had confirmed that a summary chart of his testimony was accurate.

During Durham's questioning after the defense finished, he asked Pettitte whether he remembered the conversation from 1999 or 2000 "as you described it." Pettitte said yes.

Durham asked him whether he took "a mental note" of that conversation. Pettitte said yes.

And the prosecutor asked whether any other conversation from that time stood out. Pettitte said, "No, sir."

Wednesday afternoon, Durham showed jurors a series of photographs of the used needle, cotton balls and swabs that Clemens' former trainer, and chief accuser, Brian McNamee said he saved after injecting the pitcher with steroids and HGH.

Durham said in his opening statement that physical evidence will corroborate McNamee's testimony that he injected Clemens with performance-enhancing drugs.

Pettitte's testimony of his conversation with Clemens is important, because, for the government, it represents the other key corroboration of McNamee's allegations.

Clemens' attorney Rusty Hardin told the jury during his opening statement last week that McNamee is the only witness who says he has a firsthand account of Clemens using performance-enhancing drugs, adding, "We welcome Andy Pettitte's testimony."

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