SAN FRANCISCO -- Brandon Webb can't explain it, but for some reason, when he throws poorly in the bullpen prior to a start, he seems to pitch his best games.

After he finished warming up before Tuesday's game, Webb flipped the ball to pitching coach Bryan Price and said to him, "I'll probably deal tonight."

Webb did wind up dealing, but didn't come away with a win as Moises Alou's two-run walk-off homer in the ninth led the Giants past the Diamondbacks, 4-2.

Webb, who finds himself locked in a neck-and-neck battle with the Cardinals' Chris Carpenter for the National League Cy Young Award, wound up with a no-decision, but still may have strengthened his case for the award.

While Webb was charged with two runs, just one of which was earned, over eight innings to lower his ERA to 2.88, Carpenter, who also pitched Tuesday, allowed six runs on 12 hits in seven innings. His record fell to 15-8 and his ERA increased to 3.09.

"It's too bad," D-Backs manager Bob Melvin said of Webb not getting the win. "Brandon threw the ball as well as he has all year. We just, one, didn't give him enough support and two, didn't play flawlessly behind him."

The D-Backs held a 2-1 lead heading into the eighth when Pedro Feliz hit a chopper to third that Chad Tracy couldn't handle for an error. Tracy normally would have played the ball off to the side in order to better judge the speed of it as it came towards him. But with a one-run lead, the last thing he wanted to do was let the ball scoot past him and allow Feliz to get into scoring position.

"In that situation, I just wanted to keep my body in front of it so if I don't catch it, it won't go for a double," Tracy said. "In this case, it ended up working against me. I got the in-between hop, it ate me up and I went to pick it up, [but] it rolled a little too far from me."

The error allowed Feliz to reach, and after pinch-runner Fred Lewis moved to second on Eliezer Alfonzo's sacrifice bunt, he then moved to third on a wild pitch by Webb.

The D-Backs pulled the infield in and second baseman Alberto Callaspo made a nice backhand play on pinch-hitter Mark Sweeney's grounder, but his throw to the plate short-hopped past catcher Chris Snyder.

Even if Snyder had somehow fielded the ball cleanly, he might not have been able to tag Lewis in time.

"It's a tough play," Snyder said. "But it's one I'm going to bed tonight thinking I should have made."

Once Webb got out of the inning with the score still tied, Melvin decided to go to the bullpen.

"I can't run him out there after that," Melvin said. "I mean, he's in a no-win [ninth] inning. He did his job and I wasn't going to put him in a position to take a loss."

Webb, who had thrown just 86 pitches to that point with only one ball hit in the air to the outfield, wanted to go back out for the ninth, but understood his manager's decision.

"He's looking out for me and I appreciate it, but it's sometimes tough to let somebody else go out there when you feel good and have a low pitch count," Webb said. "But we talked about it and I pretty much agree."

Randy Choate started the ninth, and after retiring the first batter he faced, the left-hander plunked Todd Linden with a pitch.

Melvin then brought in right-hander Brandon Lyon to face Alou, and after missing with his first pitch for a ball, Lyon left a changeup over the middle of the plate and Alou hit it into the seats in left.

It was the second walk-off homer of the year for Alou, with both coming at the expense of Arizona.

"I was trying to go changeup down and away and just left it out over the plate, and he did what he did with it," Lyon said. "It's a tough loss for us after Webby came in and pitched so well for us, having a chance to win there and not being able to slam the door and get a win."