The slumping outfielder drove in the winning run with a double in the 10th inning to allow the Padres to snap a six-game losing streak with a 2-1 victory over the Colorado Rockies.
The one-out double was Giles' second hard hit of the game as he finished 2-for-3 with two walks batting in the leadoff spot.
"To get a couple of hits, it helps your confidence," Giles said. "Hopefully, it just snowballs from here."
Giles' double off Matt Daley (0-1) scored pinch-hitter Edgar Gonzalez from first base. Gonzalez reached on a one-out walk and Giles hit a ball off the right center-field wall that allowed Gonzalez to score the winning run without a throw.
Giles, who came into the game hitting .153 and was on a 3-for-27 skid, was inserted into the leadoff spot for just the third time this season. The left-hander, who is annually among the NL leaders in walks and on-base percentage, hopes the win is a catalyst for San Diego, which won for only the third time in 15 games.
"It was big because looking back at the last couple of weeks, we were not getting the breaks we were getting early on," Giles said.
Manager Bud Black called it a "Brian Giles game -- hard-hit balls, two more walks, and then he squared up that last one. This is more the norm for Brian."
Heath Bell (1-0) pitched two scoreless innings, allowing one hit and striking out four for the win.
For the second straight night, the Padres set a record for their lowest attendance at Petco Park with an announced crowd of 13,646. On Monday, the same teams drew an announced crowd of 14,717 to the downtown park that opened in 2004.
Colorado has lost all eight games this season decided by one run, including three of its last five games.
Geer pitched seven shutout innings before Chris Iannetta homered into the second deck in left leading off the eighth to tie it 1-all. Iannetta hit a go-ahead grand slam off Cla Meredith on Monday night in the seventh inning of the Rockies' 9-6 win.
"I wish I could have that one pitch back," Geer said of the fastball that tailed into Iannetta. "That's really his hot zone."
Six of Iannetta's 12 hits are home runs.
Geer allowed one run and five hits in a career-best eight innings and left with his third no-decision in as many starts. Geer, who made one relief appearance, has an ERA of 2.57 in his starts.
Geer struck out three and walked two, including a second-inning pass to Troy Tulowitzki that snapped his string of 18 innings to open the season without a walk, the longest stretch this year in the majors.
"He's getting a good feel for what he needs to do to win games," Black said.
Cook allowed one run and five hits with five strikeouts and three walks in his longest outing of the season. Cook is 12-4 lifetime against San Diego, including a 5-1 record with an ERA of 1.61 in eight starts at Petco Park.
The Padres scored their first run on a bases-loaded sacrifice fly by Jody Gerut in the first.
"In the first inning, I got a couple of balls up and they made me pay," Cook said. "I worked hard to get it back down and let my defense make plays behind me."
Rockies 1B Todd Helton, who went 2-for-4, had multiple hits for the seventh time in eight games. ... Padres LF Chase Headley (sore shoulder) was scratched from the starting lineup after batting practice. ... Rockies LHP Alan Embree, who faced two batters in the ninth, moved past Charlie Hough for 31st on all-time list with 859 appearances.