WASHINGTON — Andy Pettitte, the reluctant witness who was supposed to bolster the government’s case against Roger Clemens, appears to have instead substantially aided his former teammate and friend when he readily conceded he might have misunderstood their conversation about human growth hormone.

The doubt Pettitte acknowledged on cross-examination Wednesday sounded like a significant step back from his testimony the day before that “Roger had mentioned to me that he had taken HGH.”

Clemens, a seven-time Cy Young Award winner, is accused of lying to Congress in 2008 when he said he never used steroids or HGH.

Prosecutors had hoped Pettitte, with no apparent motive to lie, would reinforce a case that otherwise relies heavily on Brian McNamee, a former strength coach for both Pettitte and Clemens who says he injected Clemens with steroids and human growth hormone.