Bumgarner looks to win his third straight start and help the Giants bounce back from their first blown save, while Samardzija hopes to lead the Cubs to their first back-to-back wins as these clubs continue a four-game set.
The series opened Thursday, with Pablo Sandoval hitting a two-run double in a four-run fourth inning as San Francisco (7-4) rallied from an early five-run deficit to win 7-6 on a foggy 40-degree day.
The temperature increased to 44 degrees as the teams played under cloudy skies Friday, and it looked like the Giants were on their way to another comeback win.
They scored three times in the ninth inning after entering with a 2-0 deficit, but Sergio Romo - who had been 6 for 6 in save opportunities - gave up two runs in the bottom half.
The elements may have played a role in the ninth as the wind was blowing out to right field when Navarro homered, but Romo gave all the credit to the Cubs backup catcher.
"I made a good pitch. He put the ball in play," Romo said. "Things happen when you put the ball in play. You tip the cap to him. No excuses."
Things are likely to calm down a bit with Bumgarner (2-0, 1.32 ERA) and Samardzija (1-1, 2.63) taking the hill. They are both among the best in the NL thus far in WHIP, with Samardzija at 0.80 and Bumgarner 0.88, and each pitcher has thrown 13 2-3 innings - also near the top of the league.
Bumgarner is 3-2 with a 2.79 ERA in six starts against the Cubs, with Alfonso Soriano going 6 for 13 with a homer and three doubles and Castro getting eight hits in 18 at-bats when facing the left-hander.
Samardzija is among the major league leaders with 22 strikeouts after registering a career-high 13 over 5 2-3 innings Sunday in Atlanta, but he gave up four hits, four runs and walked four in suffering a 5-1 loss.
"It's a double-edged sword when you strike out guys like that," Samardzija said. "Your pitch count gets up, you're showing them a lot of pitches and you're doing some different things. The pitches felt good, but I had just a couple of bad decisions there that cost me."
The right-hander struck out six in his only career start against the Giants on June 4, when he didn't receive a decision after allowing two runs and seven hits while walking five in five innings of a 3-2 loss at San Francisco.
Samardzija posted a 1.59 ERA in five previous relief appearances versus the Giants.