Pettitte allowed three hits and struck out seven over eight innings to give the Yankees a solid start against a team that won a record-equaling 116 games during the regular season. Seattle beat New York in six of their nine meetings during the regular campaign.
It was the fourth straight win in the playoffs for the Yankees, who faced elimination three times against the Oakland Athletics in the division round and survived each contest.
Pettitte outpitched Seattle starter Aaron Sele, facing the minimum number of batters in all but one inning.
Sele had to work his way out of trouble in the first inning, but the Yankees touched him for the first run of the series in the second.
Jorge Posada reached on a leadoff walk and was still there two outs later. But Alfonso Soriano singled Posada to second and Chuck Knoblauch hit a sharp grounder down the third base line and the ball glanced off the glove of a diving David Bell.
As Bell scrambled for the ball in foul territory, Posada came around to score.
The Yankees increased their lead in the fourth on the key blow of the contest. Posada led off the inning with a double and O'Neill drove him home with a low liner to right that sailed into the seats.
Seattle got one run back in the fifth, but wasted a chance for another one. Edgar Martinez opened the inning with a single and went to third on a double by Mike Cameron. John Olerud's ground out to shortstop allowed Martinez to score and sent Cameron to third, but he was stranded there when Pettitte struck out both Jay Buhner and Dan Wilson.
New York added an insurance run in the ninth against reliever Jose Paniagua. Soriano began the inning with a line drive off the left-field wall. Soriano, thinking he had hit a home run, failed to run all out and wound up having to settle for a single.
But the speedy Soriano eventually stole second and scored on a two-out single by David Justice.
Mariano Rivera came on in the ninth to record the 22nd playoff save of his career, but not without giving up a double to Ichiro Suzuki, two wild pitches that brought Suzuki around to score and a walk to Bret Boone that brought the tying run to the plate.
But Edgar Martinez grounded out to first base to end the game.