Pettitte didn't make many mistakes in his eight innings, giving up four hits with no walks, the lone run coming in the sixth. But he retired the side in the next two innings, part of six 1-2-3 frames.
"I don't know if I might have ran through it a little too quick and wore myself out early or what," said Pettitte (11-7), who earned the victory as the Yankees improved to 53-45. "More than anything, it was a hot, hot day yesterday. The guys had to play a lot of innings. I was trying as hard as I could to not make my guys stand out there all day on the field."
Thanks to Pettitte, the Yankees and their fans -- who didn't all stick around for the end of Saturday's nearly five-hour contest -- weren't out there for long on Sunday.
Not with Giambi's solo homer, which was barely fair and just over the right-field fence, hit with the first swing he took in three at-bats, no less.
Not with closer Mariano Rivera coming in and notching his 24th save.
Not with the A's (51-47) striking out nine times -- 39 Oakland players fanned in this series -- and mustering just five hits.
And not with catcher Jose Molina, who for the second straight day sent everyone home with a Yankees win on the last play. Molina was hit by a pitch with the bases loaded to seal a walk-off 4-3 victory on Saturday, and he threw out pinch-runner Rajai Davis attempting to steal second for the final out on Sunday.
"I always say, and this is my 14th year in the game, there are going to be games where you are scored upon a lot, and there are going to be games where you score a lot of runs," Giambi said. "But 2-1 ballgames, 3-1 ballgames make up your season, and those are the games you have to win to get yourself into the playoffs."
Pettitte struck out five of the first 11 hitters in the 114-pitch outing. The Yankees had a reliever warming up in the bullpen in the seventh and eighth innings, but they didn't need them.
"It was amazing," Molina said. "He was hitting his spots. He was doing everything the right way. He was throwing strike after strike. I don't even know how many first-pitch strikes we had the whole game, but I know it was a lot.
New York has won seven straight at home and eight of its last 11 overall, a welcome trend for Pettitte, who voiced his displeasure the last time he pitched, in a Yankees loss to the Blue Jays before the All-Star break, saying, "We stink right now."
"When I have a bad outing, I wear my feelings on my sleeves a little bit," he said. "I expect an awful lot out of us. I expect an awful lot out of myself.
"Maybe I was being a little harsh. Maybe I should have said, 'I stink,' more than anything. But we're the New York Yankees. I feel like we're the New York Yankees, man, and I feel like ... even with good pitching, I feel like sometimes, maybe we shouldn't always just tip our cap to everybody throwing a good game."
Meanwhile, the Yankees were complimentary toward Oakland starter Justin Duchscherer. The Major League leader in ERA yielded two runs, six hits and three walks and stuck out four.
New York got on the scoreboard in the third on Alex Rodriguez's sacrifice fly, which scored Derek Jeter. Oakland manager Bob Geren protested, to no avail, apparently arguing that Jeter left third base early before the throw from right fielder Ryan Sweeney.
Other than that, Duchscherer was sharp.
"The homer I hit wasn't really a mistake [by Duchscherer]," Giambi said of his 20th home run, which ties him with Rodriguez for the most on the team. "I just happened to get my hands to it and got him on a Yankee Stadium homer."
Next for the Yankees is another three-game home series against another playoff contender, the Minnesota Twins, on Monday. Making its own playoff push, New York has gotten off to a strong start in the second half of the season with the sweep of Oakland, and sits 4 1/2 games behind Tampa Bay in the American League East and 3 1/2 behind Boston in the Wild Card race.
The Yankees play the Red Sox after the Twins, in Boston, where Pettitte is likely to make his final July start, with all the implications and Massachusetts heat attached to it.
The Yankees believe he's up to the task.
"I've always thought Andy's been a guy you can really rely on," manager Joe Girardi said. "And I think the importance of the games after the break sometimes is magnified, and they become more and more important.
"And he seems to hit his groove."