WASHINGTON -- The government's case against Roger Clemens reaches a crucial juncture as the fourth week of the former star pitcher's federal perjury trial begins Monday.

Brian McNamee, the strength trainer who told Congress he injected Clemens with performance-enhancing drugs on numerous occasions and saved evidence he and the government contend proves it, is due on the stand early in the week for what is expected to be the most dramatic testimony of the trial.

With the defense poised to attack McNamee's credibility, decisions about the government's star witness -- and the key witness who testified last week, Yankees pitcher Andy Pettitte -- are about to land in Judge Reggie Walton's hands.

Unbeknownst to the jurors who ultimately will decide the case, the status of testimony from not only McNamee but also Pettitte will be decided by Walton after hearing arguments out of the jury's presence.

The key questions:

Just how much will the defense be allowed to delve into McNamee's "prior bad acts," as the prosecution has called them, his divorce proceedings and other aspects that could damage the credibility of the only person claiming direct knowledge of Clemens' use of performance-enhancing drugs?

Will the admission by Pettitte that he's uncertain whether he misunderstood Clemens or he in fact heard Clemens admit to taking human-growth hormone during a workout prior to the 2000 season cause his testimony about the conversation to be stricken from the trial?

The defense on Monday morning submitted its official motion to strike that part of the testimony after getting Pettitte to agree that he was 50-50 on whether he heard Clemens correctly.

"The court should not allow the jury to consider an alleged 'admission' that has all the weight of a coin flip," the defense motion read.

The government has yet to submit its written argument on the matter.

The resolution of those issues and the much-anticipated presence of McNamee in the same courtroom with Clemens -- and his defense team -- will make this a pivotal week in the proceedings.

Clemens is being tried on six federal charges of perjury, giving false statements and obstruction of Congress stemming from his Feb. 5, 2008, deposition and his Feb. 13, 2008, appearance before the House Committee on Oversight and Government Reform, during which he denied ever using performance-enhancing drugs.

McNamee testified before Congress that he injected Clemens with steroids and human-growth hormone on numerous occasions, and that he saved physical evidence he says proves it.

That evidence was shown to the jury last week when federal agent Jeff Novitzky was giving direct testimony under questioning from Assistant U.S. Attorney Steven Durham. Novitzky testified to receiving the evidence in January 2008, and he described in great detail the vials of steroids and HGH, syringes, apparently bloodstained cotton balls and other medical waste McNamee says he kept in a 16-ounce beer can since 2001.

Novitzky will be on the stand as the trial reconvenes Monday to complete redirect examination by Durham. He'll be followed by Federal Bureau of Investigation agent John Longmire, the lead case agent who has overseen the gathering of evidence against Clemens and who likely will further set up the connection between the evidence and Clemens, which the government contends was confirmed via DNA testing.

McNamee won't testify until Walton lays down the ground rules on what is relevant to the case. Lawyers for both McNamee and his ex-wife have filed motions to quash a subpoena issued by Clemens' defense team for documents from their divorce, and Walton is expected to rule on those motions sometime Monday. The defense still hopes to expand the extent of McNamee's personal and professional history that can be shown to the jury.

Last week, Pettitte's testimony went generally as planned for the prosecution until the last minutes of his cross-examination by defense attorney Michael Attanasio, who is representing Clemens along with Hardin. Attanasio asked Pettitte if it was fair to say that he's "50-50" on his recollection of the conversation about HGH, and Pettitte said, "I'd say that's fair."

As soon as Pettitte left the stand, the defense moved to strike his recollection of the conversation, and Walton suggested the prosecution did not rehabilitate its witness in redirect examination. The prosecution asked for 48 hours to respond to the motion to strike the testimony.

"My understanding is that at this time he's conflicted, he didn't know if Mr. Clemens said that to him," Walton said of Pettitte.