CINCINNATI -- There was one pitch that Marlon Byrd was hoping Sergio Romo wouldn't throw him in the eighth inning on Thursday night.

"I was just praying he didn't throw that sweeping slider, which you just can't touch," Byrd said.

The San Francisco Giants right-handed reliever did throw a slider, but it stayed out over the plate and Byrd did what he has done to most pitches throughout May -- he crushed it.

With the score tied and one out in the eighth, Byrd launched a 1-1 pitch from Romo into the left field seats, lifting the Cincinnati Reds to a 4-3 victory over the Giants in the opener of a four-game series at Great American Ball Park.

Byrd's home run was his sixth in 11 games and eighth of the season. After batting .169 with two home runs in April, the left fielder is batting over .300 with eight homers in May.

"I thank (Reds manager) Bryan Price for putting me out there after the April I had," Byrd said. "It's easy to try and go in another direction, which he could have done. But he kept faith in me. Not sure what happened in April. We just kept working."

Cincinnati (18-17) improved to 21-8 against the Giants in the past four seasons behind another solid outing from Johnny Cueto, who allowed two runs, five hits and three walks in seven innings. He struck out nine.

"Especially when your ace is on the mound, you want to get Ws," Byrd said. "Johnny gives everything he's got every time out."

Reds closer Aroldis Chapman struck out two in the ninth for his seventh save.

Jumbo Diaz (2-0) picked up the victory by retiring the final two Giants in the eighth. Romo (0-2) took the loss.

Giants starter Tim Lincecum held Cincinnati hitless through three innings but struggled with his command throughout, issuing a season-high five walks in 4 2/3 innings.

Lincecum was scored upon for the first time in May, giving up three runs and five hits while striking out four.

Twice Lincecum appeared to catch his spikes on the mound and nearly fell. He repeatedly went to the scraper at the back of the mound to clear dirt off his cleats.

"It was a little slippery, a little muddy," Lincecum said. "You've got to make the adjustment and I didn't. It mentally got a hold of me. Again, that's on me. I've just got to tighten my focus."

Lincecum escaped with a no-decision when San Francisco (17-18) tied the score 3-3 in the eighth.

Second baseman Joe Panik tripled off reliever Tony Cingrani and scored on center fielder Angel Pagan's single to right against a drawn-in infield.

It was the second time this season that Cueto's fell victim to a blown save.

Cueto, who has a 4.40 career ERA in the first inning, started slowly again, allowing a run, two hits and a walk.

In the third, San Francisco left fielder Gregor Blanco doubled and later scored from third when Cueto was called for a balk, making the score 2-0.

Cueto responded after the balk, striking out seven of 10 batters during one stretch.

"You get your chances against a guy like that, you've got to take full advantage of it," Giants manager Bruce Bochy said. "I don't think we did."

Lincecum held the Reds hitless until third baseman Todd Frazier's double leading off the fourth. Frazier scored on catcher Brayan Pena's single to center, snapping Lincecum's 18-inning scoreless streak.

The inning ended when second baseman Skip Schumaker lined hard to Pagan, who doubled Pena off first base.

In the fifth, Cincinnati loaded the bases for the second time in the game. This time, the Reds cashed in when Byrd's bloop hit scored two runs for a 3-2 lead. He finished 2-for-2 with a homer and three RBIs.

"No one's happier than Marlon," Reds manager Bryan Price said. "We knew he was the missing piece (in left field). He got off to the slow start. But he worked so diligently to get back on track."

NOTES: Cincinnati RF Jay Bruce made his 1,000th career appearance. ... Reds 2B Brandon Phillips was out of the lineup for the second straight game because of turf toe. He was available to pinch-hit, though he didn't get into the game. He likely will return Friday. ... Giants RHP Matt Cain, on the disabled list retroactive April 4 with a sore right forearm, is scheduled to throw an extended bullpen session Friday. RHP Jake Peavy, on the DL since April 18 with a strained back, threw a light bullpen Thursday. ... The Reds are 21-8 vs. the Giants the past four seasons. ... Reds manager Bryan Price said before Thursday's game that RHP Raisel Iglesias, who picked up his first win in his second career start Wednesday, will remain with the big-league club in a bullpen role.