Andy Pettitte is unsure of his conversation with Roger Clemens about HGH

For a day and a half, Andy Pettitte had been the perfect prosecution witness — he said everything the government expected of him, often answering questions with single-word responses.

He even seemed believably pained while providing incriminating testimony against his former teammate, friend and baseball legend Roger Clemens, who is being tried on charges of lying to Congress in 2008 when he claimed never to have taken performance-enhancing drugs.

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Roger Clemens’s second trial begins in D.C.: Former Major League Baseball pitching powerhouse Roger Clemens returned to the District’s federal courthouse Monday to be tried for a second time on charges that he lied to Congress about using performance-enhancing drugs.

Then, in a moment worthy of Perry Mason, Pettitte took it all back Wednesday. After some gentle prodding by an experienced defense attorney, Pettitte admitted he was no longer sure that Clemens had confided in him years ago that he had taken taken human growth hormone (HGH), a performance-enhancing substance later banned by Major League Baseball.

A day earlier, he had emphatically told jurors that very thing.

“As you sit here today, you believe in your heart and mind that you very well might have misunderstood Mr. Clemens in 1999 or 2000?” asked one of Clemens’s defense lawyers, Michael Attanasio.

“Could have,” Pettitte said.

“It’s 50-50 that you might have heard it, might have misunderstood it?”

“That’s fair,” Pettitte replied.

Pettitte’s testimony was yet another dramatic moment in the lengthy prosecution of Clemens on federal charges of perjury, obstruction of Congress and making false statements. Prosecutors allege that Clemens lied when he denied having taken steroids or HGH to Congressional investigators and a House panel in 2008.

The baseball superstar’s first trial ended last year when U.S. District Judge Reggie B. Waltondeclared a mistrial after just two days of testimony because prosecutors with the District’s U.S. Attorney’s Office presented barred evidence to jurors.

The retrial started in April, and Pettitte was just the second witness called to the stand by prosecutors.

Legal experts said Pettitte’s fuzzy memory might hurt prosecutors.

“This case is all about somebody’s recollection, and now you have the government’s star witness telling jurors his recollection could be wrong,” said Steven Levin, a defense attorney and former federal prosecutor.

Prosecutors sought to rehabilitate Pettitte’s testimony by having him verify the accuracy of a chart depicting his earlier statements, but their efforts might not have gone far enough. Defense attorneys asked the judge to strike the HGH conversation from evidence, and Walton seemed inclined to do so.

“He is conflicted,” Walton said of Pettitte, before ordering both sides to submit legal briefings on the issue. “He doesn’t know what Mr. Clemens said.”

Pettitte, a 39-year-old Texan who speaks in a drawl and is a baseball star in his own right, is considered an important witness because he has nothing to gain in testifying against his friend and mentor and has generally not wavered in what he told Congressional investigators in 2008.

At that time, Pettitte said “The Rocket” had confided to him during a workout at Clemens’s home gym in 1999 or 2000 that he had taken HGH. Pettitte even submitted an affidavit to that effect.