PHOENIX — Tim Lincecum might be a 20-game winner in form and fairness, but not in practice.

Not yet, anyway.

The Giants' bullpen has blown five of Lincecum's leads this season and left fielder Eugenio Velez essentially tanked another Thursday night, costing Lincecum a shot at becoming the Giants' first 20-game winner since 1993.

Instead, the Arizona Diamondbacks handed Lincecum his first loss since July 20 while preserving their own playoff lives in a 3-2 victory over the Giants at Chase Field.

Nothing has come easily for the Giants this season, not even for their twirling prodigy. Lincecum (17-4) suffered a bloodied left index finger when a sacrifice bunt attempt went awry against Randy Johnson, but he stayed in the game and held a one-run lead in the sixth inning. That's when Velez misjudged Justin Upton's flare to left field, breaking in on a ball that went over his head for a two-run triple.

Bengie Molina's solo homer tied it in the eighth and sent the Giants' dugout into an uproar, but Adam Dunn hit a run-scoring single off a tiring Lincecum in the bottom of the inning as Arizona swept the four-game series.

The game had the feel of a one-game playoff, and the Giants were desperate to get Lincecum another victory to perhaps put a Cy Young Award on ice.

"Frustrated, maybe, is the biggest word," Molina said. "Not only frustration for Timmy, but for the team losing four here. However you want to put it, that's not a way to lose the game."

Lincecum struck a similar note.

"We didn't want to play the Diamondbacks like we did," said the right-hander, who threw 118 pitches in tossing a second consecutive complete game. Everybody's taking this pretty hard and we should."

Before the game, Giants Manager Bruce Bochy announced he'd keep Lincecum on his regular day and push back the rest of the rotation, lining up his ace to pitch Tuesday against the Colorado Rockies and in the Sept. 28 season finale against the Los Angeles Dodgers.

With no chance at 20 wins, why do it?

"He's throwing the ball great, he feels great, so why not?" said Bochy, who made the call after a summit with G.M. Brian Sabean and trainer Dave Groeschner.

Bochy acknowledged the value in ending this season on an emphatic note behind their Cy Young candidate. Left unsaid: that those good vibes could influence season-ticket holders who are on the fence about re-upping for 2009.

Bochy did say he'd watch Lincecum's pitch count, after he threw a career-high 138 in his previous start. Lincecum's 3,502 pitches are the most in the National League.

Lincecum survived a scare in the fifth, when he squared to bunt and Johnson threw a fastball that rode up and in. Lincecum recoiled as the ball struck his left index finger, which was wrapped around the bat.

"Maybe if I worked on my bunting technique," said Lincecum, who went for X-rays that did not reveal a fracture. "I'm over the plate giving him two targets — the plate and me."

Stephen Drew led off the sixth with a double over center fielder Aaron Rowand's head and Conor Jackson was hit by a pitch. But Lincecum struck out Dunn and was poised to escape when he threw a two-strike change-up to Upton that hovered inches above the dirt.

"He barreled it up," Lincecum said. "Good pitch, better swing."

It was a defensive swing that often fools outfielders into charging the ball, and Velez was easily duped. He was making just his fourth start at the position this season.

"It was right there," said Velez, who jumped but got only a piece of the ball. "That line drive "& I've never seen that ball before. When I see the swing, I think it's going to be like a blooper."

It was a delicious twist for the Diamondbacks, who had their playoff dreams dented when Velez poked consecutive game-ending hits Sept. 9-10 at AT and T Park.