CINCINNATI -- Jeff Samardzija dominated Cincinnati Reds batters for much of his outing Tuesday night. However, it was his feat of strength in the eighth inning that both impressed and concerned San Francisco Giants manager Bruce Bochy.

After striking out swinging on a slider to end the eighth, Samardzija snapped the bat in two over his left leg, then tossed the splintered remnants aside.

"That's just how competitive he is," Bochy said. "I think we were all stunned. You don't see that from a pitcher. There's the chance of (injuring) the thigh or knee there. But the way that thing broke off, he looked like Herman Munster up there."

Samardzija (4-1) allowed one run over eight innings, and Buster Posey doubled three times and scored a run, lifting the Giants to a 3-1 victory over the Reds at Great American Ball Park. He gave up three hits with two walks and nine strikeouts over 115 pitches.

The Giants right-hander came in with four hits in 16 at-bats, but he went 0-for-4 with two strikeouts Tuesday.

"I didn't have too many good at-bats out there," said Samardzija, who said he felt no pain from the bat's impact. "No premeditation there. Go with what you feel. It is what it is. Stupid."

Santiago Casilla earned his sixth save.

Adam Duvall, who was acquired from the Giants in the trade for Mike Leake, homered for Cincinnati (10-17) to tie the score 1-1 in the fifth.

However, the Reds' bullpen allowed a run in a 21st consecutive game to establish a modern-day major league record. The 2013 Rockies bullpen gave up a run in 20 straight contests.

The record-breaking run scored in the eighth. Hunter Pence walked and later raced home from third on Brandon Crawford's infield dribbler to put the Giants (15-13) ahead 2-1. Gregor Blanco followed with an RBI single to make it a two-run lead.

"It's a challenge for the guys in the bullpen," Cincinnati manager Bryan Price said. "There's no wiggle room for the bullpen to make a mistake."

It could have been a bigger inning for the Giants had Brandon Belt not been thrown out trying for a double for the second out, which was confirmed by video review.

"It was a bang-bang play," Bochy said. "We needed (those runs), too, because their kid (Reds starter John Lamb) did a nice job, too."

Cincinnati reliever Caleb Cotham (0-1) allowed two runs and three hits in one inning.

Posey went 3-for-4 with three doubles, a walk and a run for San Francisco. It was his second career three-double game.

Lamb made his season debut for the Reds in place of Jon Moscot, who was scratched due to shoulder discomfort. Lamb responded with six solid innings of one-run ball. He gave up six hits and three walks while striking out four.

Lamb began the season on the disabled list as he recovered from back surgery performed in December. Although he felt fine physically, Lamb wasn't pleased with his three rehab starts that resulted in a 9.22 ERA.

It was more of the same early in Tuesday's game when he allowed back-to-back doubles by Posey and Pence to put the Giants up 1-0 in the first.

Lamb threw 22 pitches in the first inning.

Despite five doubles among their first six hits off Lamb, the Giants managed only a run.

"I had mixed emotions," Lamb said of his outing. "I'd like to go out there and be textbook and get ahead. At the end of the day, I was able to use my secondary pitches to get guys out and get the game in my favor. I had no issues with my back."

San Francisco ran itself out of a scoring chance in the fifth when, with runners on first and second, Denard Span was caught stealing in a rundown between second and third for the final out.

Duvall took a 1-2 pitch from Samardzija out to left-center to tie the score in the fifth.

"Cutter up and away, could've thrown it off the plate a little more," Samardzija said. "We were pounding the zone. Great American 'Small' Park got me."

NOTES: Reds LHP John Lamb was recalled from Triple-A Louisville to start Tuesday night in place of RHP Jon Moscot, who has discomfort in his non-throwing shoulder. Moscot could be available to pitch out of the bullpen within a day or two. ... Lamb is the ninth starting pitcher used by Cincinnati in the first 27 games, matching the franchise high last occurring in 1972. ... Cincinnati also optioned RHP Drew Hayes to Triple-A on Tuesday. ... Giants 2B Joe Panik, who is nursing a groin injury, hit in the cage Tuesday and took ground balls. He could return by Thursday. ... Giants LF Angel Pagan missed his second game in a row due to a mild hamstring strain.