Washington (CNN) -- For baseball fans, July signals the midpoint of America's pastime: the All-Star Game, the full swing of pennant races.

But this year it also marks the start of a perjury trial against one of the greatest players in the sport's history.

Roger Clemens, whose career feature seven Cy Young Awards and the ninth-most all-time wins among pitchers, goes on trial this week in a federal court just blocks from the U.S. Capitol, where he allegedly lied to Congress in 2008. Jury selection is scheduled to begin on Wednesday.

Clemens, 48, has pleaded not guilty to a six-count indictment and denied he ever used steroids during this career. He was charged with one count of obstruction of justice, three counts of making false statements and two counts of perjury.

Back on February 13, 2008, Clemens was called to testify before the House Committee on Oversight and Government Reform. Clemens steadfastly denied ever using steroids in dramatic testimony before lawmakers. However, what he said was starkly at odds with testimony given at the hearing by his former personal trainer, Brian McNamee, sitting at the same witness table.

McNamee, who served as Clemens' trainer until 2007 -- the year Clemens retired from baseball -- testified he kept soiled medical dressings, containers, hypodermic needles and other materials allegedly used to give illegal performance-enhancing drugs to the baseball legend.

"They both insist they are telling the truth, but their accounts couldn't be more different. They don't disagree on a phone call or one meeting. They disagree over whether a period of over four years Mr. McNamee repeatedly injected Mr. Clemens with steroid and human growth hormone," Rep. Henry Waxman, D-California, said at the 2008 hearing. "It's impossible to believe this is a simple misunderstanding. Someone isn't telling the truth."

"Baseball is what I do. It's not who I am. I played the game because of my love and respect for it," Clemens said in his opening statement. "I've devoted my life to it and pride myself an example for kids -- my own as well as others... Let me be clear: I have never taken steroids or HGH (human growth hormone)." He said, "If I'm guilty of anything, it is of being too trusting of everyone, wanting to see the best in everyone, and being too nice to everyone. If I'm considered to be ignorant of that, then so be it."

Clemens insisted he only received vitamin B-12 shots from McNamee, but the ex-trainer insisted every injection contained steroids or other performance enhancers.

Waxman said pointedly, "If Mr. McNamee is lying, then he has acted inexcusably and he has made Mr. Clemens an innocent victim. If Mr. Clemens isn't telling the truth, then he has acted shamefully and he has smeared Mr. McNamee. I don't think there is anything in between."

Clemens' testimony was also in contrast to a sworn affidavit provided by New York Yankees pitcher and close friend Andy Pettitte, who told the committee that Clemens in 1999 or 2000 had taken HGH.

"Is this true?" asked Rep. Elijah Cummings, D- Maryland.

"It is not," replied Clemens.

"So you did not tell Mr. Pettitte that you used human growth hormone?"

"I did not."

"But at the same time, you just said that he's a very honest fellow. Is that right?"

"I believe Andy to be a very honest fellow, yes."

Cummings pressed Clemens, reminding him that he was under oath.

"Mr. Clemens, do you think Mr. Pettitte was lying when he told the committee that you admitted using human growth hormones?"

"Mr. Congressman, Andy Pettitte is my friend. He will be my -- he was my friend before this. He will be my friend after this. And again, I think Andy has misheard," Clemens replied.

McNamee insisted he injected Clemens and other players on numerous occasions.

"I want to be clear that what I did was wrong," McNamee testified. "I want to apologize to the committee and to the American people for my conduct. I have helped taint our national pastime. I hope that my testimony here today allows me, in some small way, to be part of the solution. I'm not proud of what I have done, and I am not proud to testify against a man I once admired... I have no reason to lie and every reason not to. If I do lie, I will be prosecuted," he added.

However, McNamee's credibility has also been called into question. Rep. John Tierney, D-Massachusetts, pressed McNamee at the hearing, "Why did you mislead the (federal) investigators?"

McNamee explained, "The part about injections were part recollection and part withholding, trying not to hurt these players ... But everything I told them about their use was true."

McNamee testified he told investigators he had "no direct evidence as far as physical evidence."

"So you didn't tell the truth, then, initially, to them?" asked Rep. Dan Burton, R-Indiana.

"No, sir," replied McNamee.

"You lied," Burton shot back.

"Yes, sir."

On Wednesday, a federal district court jury will begin to decide who is telling the truth.

Guilty or not guilty, Clemens, nicknamed "the Rocket" during his 24-year career, has undoubtedly left the game with some questionable calls.