Stewart, who has spent most of his career as a regular player, came off the bench in the 10th inning and delivered a single that plated the two deciding runs in the A's 5-3 win.
"I don't have much experience at it, and anybody that's a starter usually doesn't pinch-hit too much," Stewart said. "It's really hard to sit for eight or nine innings and get a hit off a closer or a tough pitcher."
But that's exactly what he did, taking Brad Hennessey's 2-2 slider and hitting a sinking liner into left that scored Mark Kotsay and Jason Kendall.
"I was just trying to make some type of contact, and I told myself to cut down on my swing and go easy," Stewart said.
"That's a tough spot," manager Bob Geren said. "He's done a great job for us, and he continues to help the team. We gave him plenty of notice, and he got ready and got a hit."
Prior to June, Stewart had started in 49 of the A's 52 games, but he has not been in the starting lineup in five of the nine games this month after Saturday's game in San Francisco.
The A's have four very good outfielders, with Travis Buck, Kotsay and Nick Swisher joining Stewart.
"I still want to try and mix it up and keep everybody fresh," Geren said. "We have a lot of good players."
One of the games Stewart did start this month was Thursday's home finale with Boston, in which he came up with another big hit.
With two outs in the bottom of the ninth, Stewart singled to right and broke up Curt Schilling's bid for a no-hitter.
Buck lights up Lincecum: Buck got to see a lot of Giants rookie right-hander Tim Lincecum when the two were playing college baseball in the Pac-10.
Buck, who attended Arizona State, saw Lincecum when the pitcher was at Washington. Buck didn't recall exactly how he did off Lincecum, but "probably not as good as I did tonight," he said after Friday's game.
Buck faced Lincecum three times and had three hits, including two doubles. He went on to have his first career four-hit game.
"[Lincecum's] gotten a lot better since the last time I saw him," Buck said. "He's got a bright future ahead of him."
The four hits Buck had from the leadoff spot were more than he had from that spot during his entire college career. He had three career leadoff hits as a Sun Devil.
"He has some characteristics of any of those hitters -- one, two, three and four," Geren said. "He has plate discipline, he has good power and he can run real well."
"[Where I hit] is just a number," Buck said. "I look at it like, you only lead off once, at the beginning of the game. I'm not going to change anything. I'm going to go out there and keep the same approach I've always had."
Murphy bruised but OK: When Donnie Murphy collided at home plate with Giants catcher Eliezer Alfonzo in the 10th inning of Friday's game, it appeared that both of them might be seriously hurt.
It turns out that the smaller of the two, and the one who wasn't wearing any protective gear, is still standing.
The 5-foot-10, 185-pound Murphy arrived at the stadium before Saturday's game with scrapes on his right leg and was a bit swollen and sore.
"We had a contact play on, and both of us had nowhere to go," Murphy said of the collision. "At first I saw the outside of the plate open, so I tried to go there."
It was the 5-foot-11, 210-pound Alfonzo who took the brunt of the punishment after blocking the plate. Alfonzo is now on the disabled list with a sprained left knee, but it is feared he might have a torn posterior cruciate ligament. If so, he will need surgery and will be out for the year; if not, it is estimated he will be out for two months.
Murphy, on the other hand, was listed as available for Saturday's game.
Dribblers: Geren, who loaded the Athletics' lineup with left-handed bats the first time the team faced Barry Zito, did the same in Saturday's game, using four. Lefties are hitting .300 off Zito this year, righties only .214. ... Rich Harden threw a bullpen session from flat ground on Friday and might throw from halfway up the mound on Sunday. "He's progressing, and he said he feels great," Geren said.
Up next: Lefty Lenny DiNardo (2-2, 1.45 ERA) will take the ball opposite Giants righty Matt Cain (2-5, 3.54 ERA) in Sunday's series finale. The first pitch is set for 1:05 p.m. PT.
Matt Smith is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.