Pettitte will take the mound Sunday at Yankee Stadium with a chance to clinch a World Series berth if he can defeat the Los Angeles Angels of Anaheim and Joe Saunders.
Girardi told the media Saturday that he was not considering going with another starter in Game 6 after having to put Pettitte on ice for 24 hours.
"We think the extra day could help him physically," Girardi said. "He was on short day the last time. He's been good on extra day too. So I don't really worry about him going on an extra day."
The series winner will face Philadelphia in the Fall Classic.
If the Angels win Sunday, Game 7 will be played Monday.
Angels skipper Mike Scioscia said Saunders would start Sunday and starters John Lackey, Jered Weaver and Scott Kazmir would be available in the bullpen.