Pettitte says he might have misunderstood conversation with Clemens about human growth hormone Pitcher Andy Pettitte acknowledged under cross-examination Wednesday that he might have misunderstood Roger Clemens when Pettitte claimed he heard his former teammate say he used the human growth hormone. Asked by Clemens' lawyer if it was fair to say it was "50-50" that Pettitte misunderstood the conversation from about a dozen years ago, Pettitte responded, "I'd say that's fair." As you sit here today, do you believe in your mind that you very well might have misunderstood Clemens in 1999 or 2000, right? Clemens' attorney asked. "I could have, right," Pettitte responded. Pettitte is a key government witness as the prosecution tries to prove that Clemens lied to Congress when he denied using performance-enhancing drugs. Pettitte's concession could weaken the government case. On Tuesday, Pettitte had testified, "Roger had mentioned to me that he had taken HGH." Pettitte had made the same claim in a congressional deposition in 2008, prompting Clemens to say that his friend "misremembers" their conversation. The two men arrived at opposite ends of the courthouse a few minutes apart Wednesday morning, both in gray suits. Clemens lawyer Michael Attanasio started the doubt campaign late Tuesday when he coaxed Pettitte into agreeing that Clemens' remark was a passing comment made during a workout. Attanasio also got Pettitte to praise Clemens' work ethic, mechanics and concentration -- not to mention the seven Cy Young awards he had won for his outstanding pitching. The cross-examination got to feel so much like a Clemens infomercial that prosecutor Steven Durham objected at one point. Pettitte is crucial to a government case that otherwise will rely heavily on the testimony of Brian McNamee, who worked as a strength coach for both Clemens and Pettitte and has said he injected both men with performance-enhancing substances. The government showed the jury photos of the three working out together in Texas during happier times -- "Mac, Roger and me," as Pettitte put it. Pettitte has acknowledged he received HGH from McNamee; Clemens has not. Pettitte told the jury about the time he used HGH in 2002 while recovering from an injury, but he wasn't allowed to say he was injected by McNamee because the judge earlier ruled that information inadmissible. Pettitte said he used HGH one other time, in 2004. He said he regretted it both times he tried it, that he doesn't think it helped him physically and that it has tarnished his name. "I wish I never would've" taken HGH, he said in his slow Texas drawl. If I hadn't done it, I wouldn't be here today. Pettitte also recalled the other time he spoke with Clemens about HGH, during the media swirl surrounding earlier congressional hearings -- in 2005 -- on drug use in sports. Both were playing for the Houston Astros, and Pettitte asked Clemens at spring training what Clemens would say if asked by reporters about HGH use. Clemens responded, "What are you talking about?" according to Pettitte, and said Pettitte must have misunderstood the earlier conversation, said to have taken place in 1999 or 2000. "He said, `My wife used it,"' Pettitte said. "Obviously I was a little flustered," Pettitte said, "because I thought that he told me that he did." Both Clemens and McNamee have said McNamee injected Debbie Clemens with HGH at the Clemens home in 2003, although they differ over certain details. Pettitte's appearance Tuesday came a day after he allowed six runs and 10 hits with eight strikeouts over 5-2/3 innings in an extended spring training game in Clearwater, Fla., as part his comeback attempt at age 39 with the New York Yankees. During prosecutor Durham's questioning, Pettitte described how he admired Clemens as a youngster and considered him a mentor when they played together for the Yankees and Astros. Pettitte said he still considered Clemens a good friend but hasn't been able to talk to him for a long time because of the case. He also said it was difficult to testify against his friend. But there was almost no interaction between the two large men Tuesday. About the only time Pettitte looked in Clemens' direction was when the prosecutor asked whether Clemens was in the courtroom, and Pettitte pointed to the man in the suit and "greenish tie." Clemens stood and nodded. For his part, Clemens took more notes than usual on his yellow legal pad. Fox News' Mike Levine and The Associated Press contributed to this report.