By Ian Simpson

WASHINGTON (Reuters) - New York Yankees pitcher Andy Pettitte took the witness stand on Tuesday to testify against former teammate Roger Clemens, charged with lying to Congress about his alleged use of performance enhancing drugs.

Pettitte, one of the most anticipated witnesses in the federal perjury trial, was a longtime friend of Clemens and used the same personal trainer, Brian McNamee.

Pettitte has admitted using human growth hormones in 2002 and 2004. He also testified to a congressional committee that Clemens told him in 1999 or 2000 and then again in 2005 that he had used human growth hormones. Clemens has said that while Pettitte was a close friend, he "misremembers" the conversations.

Clemens, 49, is being tried for a second time on federal charges of lying to the House of Representatives Committee on Oversight and Government Reform in 2008 about whether he used anabolic steroids and human growth hormone. The panel was investigating drug use in Big League baseball.

The trial of Clemens, a seven-time Cy Young Award winner, is expected to last about six weeks.

Clemens first went on trial last July, when Judge Reggie Walton declared a mistrial because prosecutors showed jurors a video clip that included material he had banned from the case unless it was raised by Clemens' defense team.

Pettitte has come out of retirement to bolster the Yankees' pitching staff and is playing his way back into shape with a Yankee farm club.

(Editing by Paul Thomasch and Philip Barbara)