Cook returned from a monthlong stay on the disabled list to win again in San Diego and the Colorado Rockies handed the reeling NL West-leading Padres their season-high eighth straight loss, 4-3 on Friday night.
San Diego kept its three-game lead over the San Francisco Giants, who lost 4-2 at the Los Angeles Dodgers.
The Padres have been outscored 45-19 during their losing streak, which is their longest since they lost eight straight in June 2008. The Padres lost 99 games that season.
This was the opener of a 10-game homestand against division rivals. The Padres will play two more against the Rockies, three against the Dodgers and four against the Giants.
Cook improved to 7-1 with a 2.21 ERA in 11 starts at Petco Park.
"I feel comfortable here," Cook said. "It's always 68 to 72, sunny and humid. so I know exactly what I'm getting when I come here. I love pitching here and guys on our team love playing here. It's not only the pitching that comes together here, guys seem to love hitting here, and we put it all together."
Cook (5-8) held the Padres to two runs and four hits in 6 1-3 innings. Cook, who had been sidelined by a sprained right big toe, got 15 of his 19 on groundballs.
"I felt like myself out there, which was good for a change," said Cook, who overall is 14-4 with a 2.86 ERA in 25 career appearances against San Diego, including 23 starts.
"He was tough," Yorvit Torrealba said. "But that's how he is. When he gets on a roll, he's tough to beat."
The Rockies, trying to climb back into the wild-card race, beat the Padres for the ninth time in 13 games this year. After Tulowitzki's third-inning homer, Luebke (0-1) and three relievers retired 19 straight batters until Ernesto Frieri walked Clint Barmes with one out in the ninth.
Nick Hundley had a pinch-hit, two-run homer with one out in the seventh off reliever Samuel Deduno to pull the Padres to 4-3. San Diego had runners on first and second with two outs before Ryan Ludwick struck out.
"I really believe, as soon as we win one game, we're going to get a winning streak going," Torrealba said. "Every team goes through this."
Huston Street pitched the ninth for his 14th save in 18 chances.
Luebke got out of the first despite a leadoff walk and his throwing error, but he wasn't as lucky in the second. Melvin Mora hit a leadoff triple to the left-center gap and Giambi drove Luebke's next pitch into the sand play area beyond the fence in right-center for a 2-0 lead. It was Giambi's fourth. Luebke hit Ryan Spilborghs with a pitch and walked Dexter Fowler with two outs, but Spilborghs was thrown out at the plate by left fielder Chris Denorfia on Jonathan Herrera's single to end the inning.
Carlos Gonzalez singled leading off the third and Tulowitzki hit Luebke's next pitch over the fence in straightaway center, his 13th, for a 4-0 lead.
Luebke, whose contract was selected from Triple-A Portland on Wednesday, retired his last nine batters. He allowed four runs and five hits in five innings, struck out three and walked two.
"I was disappointed because I think I should have done better," he said. "But I had a little adrenaline going. I was kind of rushing things and wasn't hitting my spots."
Manager Bud Black said he hasn't decided if Luebke will get another start.
"His stuff was good," Black said. "I think you could tell he was a little bit nervous, as expected."
Cook walked the bases loaded with one out in the sixth but the Padres scored only one run, thanks to some great defense. Giambi, the first baseman, robbed Chase Headley of what could have been a two-run single when he dove to his right for the ball, got up and ran a few steps and dove headfirst to tag the bag with his glove for the second out as David Eckstein scored. Will Venable then popped up to end the inning.
"That was awesome," Cook said of Giambi's play. "I kind of stumbled coming off the mound so I wasn't going to get to first. He dove for it so I was just hoping he could get to his feet. That was a huge play to keep the inning where it was and we were able to get back out of it. Pretty much, it was a game-saver."
Cook walked four and struck out three. Adrian Gonzalez had three hits for San Diego.