Jurors in the perjury trial of Roger Clemens should not be allowed to consider Andy Pettitte’s foggy memory of his conversation with the baseball star about performance-enhancing drugs because it has “all the weight of a coin flip,” defense attorneys said Monday.

Federal prosecutors were counting on Pettitte as a critical witness in their effort to show that Clemens lied to Congress in 2008 when he denied using steroids and human growth hormone. Pettitte initially testified last week that his former teammate and friend had confided in him about his use of human growth hormone during a workout in 1999 or 2000.

View Photo Gallery: The trial of Roger Clemens.

But Pettitte backed away from his story the next day. Under cross-examination, he conceded that he might have misunderstood Clemens and that there was a 50 percent chance that he got it wrong.

In urging U.S. District Judge Reggie D. Walton to throw out Pettitte’s testimony, Clemens’s lawyers said in a new court filing that Assistant U.S. Attorney Steven Durham “never directly confronted Mr. Pettitte’s uncertain testimony, instead skirting the critical issue.”

Walton himself noted that Pettitte appeared conflicted and said that he would have expected prosecutors to follow up more directly to try to clarify Pettitte’s current recollection of the conversation.

Walton is expected to rule on Pettitte’s testimony later this week, once federal prosecutors have responded to the defense team’s request.

In the courtroom Monday, federal prosecutors questioned federal agent Jeff Novitzky to try to counter the impression that the government was “out to get” Clemens as his defense attorney Rusty Hardin suggested last week.

Novitzky, now an agent with the Food and Drug Administration, said he was pursuing alleged suppliers to Major League Baseball and other professional sports because of the widespread use of performance-enhancing drugs and the concern that the practice was influencing young athletes.

“We asked about many different people, many different athletes,” Novitzky said, adding that Clemens was “never targeted.”

Clemens’s former strength coach, Brian McNamee, is expected to testify as soon as Tuesday that he injected the seven-time Cy Young Award winner with performance-enhancing drugs on several occasions. McNamee turned over needles, empty steroids vials and other medical waste to Novitzky and agreed to be interviewed by former senator George Mitchell for his report on the proliferation of steroids in baseball.

Novitzky portrayed McNamee as a reluctant participant in Mitchell’s investigation, describing him as nervous, late for his interview and feeling ill.

In follow-up questions with Novitzky, federal prosecutors also tried to explain why a shipping receipt for McNamee addressed to Clemens’s home turned up more than two years after his colleagues searched the home of a steroid supplier. Novitzky said the receipt was hidden under a television set that was not lifted during an initial search.

Jurors also heard Monday from special agent John Longmire of the Federal Bureau of Investigation, who described how the physical evidence was handled and how Clemens voluntarily provided a DNA sample.