Giambi connected for a pinch-hit, two-run homer with two outs in the bottom of the ninth inning and Colorado beat the Arizona Diamondbacks 4-2 on Sunday to extend its winning streak to 10 games.
"Streaks likes this, you win in all kinds of different ways," Rockies manager Jim Tracy said.
The Rockies pulled within 1 1/2 games of first place in the NL West. San Diego and San Francisco are tied atop the division after the Giants beat the Padres 6-1 on Sunday behind Tim Lincecum.
San Diego comes into Coors Field for a three-game series starting Monday night. With a few more wins and some help from the Los Angeles Dodgers, who play the Giants beginning on Tuesday, the Rockies could soon move into first place.
"That's been a goal and now we've got a shot to do it," shortstop Troy Tulowitzki said.
Jonathan Herrera started the winning rally when he reached on an error by second baseman Kelly Johnson. Clint Barmes struck out for the second out before Giambi, who was 0 for 16 against Arizona this year, drove a 1-1 pitch from Sam Demel (1-1) to center for his sixth homer of the season.
"I got a pitch I could handle and I hit it out of the ballpark," Giambi said. "I was hoping to take a good at-bat and get one in the gap because we've got Herrera at first who could score. I was lucky enough to get it up in the air."
The Rockies didn't know it was gone until center fielder Chris Young stopped running.
"Coors Field, anything's possible," Demel said. "He caught it well, got out in front, popped it up in the air and it got out of here."
Huston Street (3-4) pitched a perfect ninth for the win.
Arizona starter Ian Kennedy was masterful for six scoreless innings, giving up just two hits, striking out six and walking three. After allowing a two-out single to Tulowitzki in the first and walking Todd Helton, Kennedy retired the next 15 batters before yielding a two-out double to Carlos Gonzalez in the sixth.
Kennedy also had three of the Diamondbacks' eight hits, but his potent bat caused him to leave after throwing only 82 pitches due to a cramp in his right calf that flared up while he was running the bases in the sixth.
"I wanted to go back out there [for the seventh] because I knew my pitch count was low and my arm felt good," he said. "I was on base too much, I guess. That's usually not my problem."
Still, the right-hander was in line for his sixth win in his last seven starts when, for the second straight game, the Rockies rallied. Helton drew a leadoff walk from Blaine Boyer in the seventh and two outs later Miguel Olivo walked. Pinch-hitter Ryan Spilborghs hit a bloop double off the outstretched glove of shortstop Stephen Drew in shallow left field to drive in both runners and tie the score at 2.
"Two four-pitch walks," Arizona manager Kirk Gibson said. "You can't do that to Colorado. Both scored. That's not good baseball."
The Diamondbacks took the lead in the third with the help of an error by Colorado starter Jhoulys Chacin. Kennedy led off with a single and, after Drew struck out, Chacin threw Young's potential double-play grounder into center field to put runners at the corners.
Chacin loaded the bases again in the fourth but got Johnson to ground out, ending the inning. Chacin gave up two unearned runs in four innings, struck out four and walked four.
The Rockies had a chance to score first when they loaded the bases in the first, but Melvin Mora grounded out to end the threat.
Rockies C Chris Iannetta left after the first inning with a right calf strain. ... Diamondbacks 3B Mark Reynolds was ejected in the sixth for arguing balls and strikes with plate umpire Gerry Davis. ... Arizona OF Justin Upton (left shoulder soreness) was not in the starting lineup.