Houston was being one-hit through the first six frames, including 4 1/3 by young right-hander Tim Lincecum. It scored two runs in the third inning as Brad Ausmus walked and scored on a Darin Erstad double, and Erstad scored on a fielding error by second baseman Emmanuel Burriss.
"He's tough," Lance Berkman said of the starter. "If he's not the best young pitcher in the league, then I don't know who is. He's very difficult with several different pitches, an explosive fastball, a tough curveball that keeps you off balance."
But an unfortunate, yet key, at-bat by Ausmus in the fifth inning changed everything. Ausmus hit a line drive off Lincecum's right knee, taking him out of the game. X-rays were negative, but the loss of the starter proved costly for the Giants, whose bullpen held out for two innings before disaster struck.
The seventh inning started with an out by Michael Bourn followed by a walk by Ausmus. Hunter Pence, pinch-hitting for pitcher Roy Oswalt, singled to left, and Mark Loretta walked to load the bases.
Erstad hit a single to shallow left on two strikes, and Ausmus and Pence scored at nearly the same time, both beating the throw home from Fred Lewis to Bengie Molina.
The play at the plate was eerily similar to one a few weeks ago against the Mets, when Pence slid into Loretta's back to score two runs. Ausmus went back to tag third, and he said by the time he took off, Pence was already in his pocket.
"We're not sure if it's bad baserunning or lucky baserunning, but it worked out," Pence said. "I just read that it was going to fall in, but the runner at third in that situation has to make sure that they don't catch it, because they're going to score anyway."
Miguel Tejada then singled to right to reload the bases for a grand slam by Berkman off the left-field wall. The homer was just his second in the past 36 games and his fifth career grand slam.
Astros manager Cecil Cooper said that at-bat finally allowed him to relax, although the hitters before Berkman really made it happen.
"I just knew we were going to get some more right there, and he put a good swing on it," Cooper said. "That was definitely a big swing for us. I thought Erstad's at-bat was just key for us. He battled a tough left-hander on two strikes and got a base hit. Loretta had a great at-bat, a walk."
But the Astros were not finished. Geoff Blum hit a double to deep left to bring up Ty Wigginton, who hit a two-run homer to the front row in right field.
They notched two more in the eighth, a home run to the Crawford Boxes by Pence and a three-base score by Loretta, who singled to center before scoring on a Tejada double.
Pence had the day off because Cooper said he had noticed some frustration at the plate in the right fielder, but he showed he was still strong in the clutch.
Cooper said Oswalt did not have the best outing but was able to battle through some tough innings, leaving nine runners stranded.
"I didn't have really any kind of breaking pitch today," Oswalt said. "I just tried to battle with my fastball, tried to move my fastball around and put some different speeds on it. I got some key outs when I needed to, but I thought I gave up one too many for a while there."
Oswalt gave up three runs on 10 hits, walking three and striking out four.
Cooper walked to the mound in the seventh with two runners on and one out with the thought of taking Oswalt out, but the righty convinced him to let him finish the inning.
"I wanted to just check with him," Cooper said. "He's the guy that I think deserves that, and he said, 'I'm going to get him [John Bowker], don't worry.' I said, 'OK, you got enough left?' and he said, 'I'm fine.' So I'm going to let him have it."
Oswalt said he hoped to get a double play off Bowker but ended up striking him out, and when he expected Rich Aurilia to be aggressive on a fastball in the next at-bat, he threw a slider. Aurilia grounded into a force play at second.
Cooper said before the game that the goal was to get Lincecum out on a high pitch count. Although he left a different way, Cooper said he knew that was Houston's chance to catch back up.
The Giants' bullpen had to pick up 4 2/3 innings of work, using five pitchers.
"It didn't feel like a laugher really at any point in the game until real late," Berkman said. "It was a weird game. It started out like a pitchers' duel and ended up like a blowout."
But the Astros said they wished the Giants' starter had not gone down.
"I hope he's all right," Oswalt said. "He's a great competitor, and I like the way he goes about his business. One thing is he's aggressive with his strike zone."
Krysten Oliphant is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.