SAN FRANCISCO – Johnny Cueto warned the Cincinnati Reds after his warmup that he had experienced a sharp pain on the right side of his back when he made his final two throws in the bullpen.

Cueto lasted just eight pitches after that. He left his Game 1 start against San Francisco on Saturday night in the first inning because of back spasms, then the Reds hung on to beat the Giants 5-2 behind their stellar bullpen.

“I feel really proud about my team. I feel proud about the pitchers,” Cueto said. “They all came in and did their job.”

The Reds said he is day to day. The right-hander threw a second strike to No. 2 hitter Marco Scutaro and walked off the mound in obvious pain. A trainer and manager Dusty Baker rushed out to check on him, and the 19-game winner came out moments later.

Now, the Reds hope to get him back this series. Cueto wanted to see how he responds.

“I really feel better right now,” Cueto said after the game. “This is my first time it happened.”

He retired leadoff man Angel Pagan on a strikeout. During the at-bat, Pagan stepped out of the batter’s box and was granted time. Cueto apparently didn’t see it and continued his motion, but Baker said pitching coach Bryan Price had alerted him to Cueto’s discomfort beforehand.

The Reds turned to winning pitcher Sam LeCure and got four innings of relief from Mat Latos on three days’ rest for the first time in his career, followed by three more relievers.

Cueto threw eight pitches and six strikes for the earliest postseason exit by a starter since Atlanta’s John Thomson lasted one-third of an inning in Game 3 of a 2004 NL division series against Houston. Thomson came out with a sore muscle in his left side.

“When Johnny went down, I was like: ‘Gosh, oh no, we’re done. Why? Why? Why?’ ” Brandon Phillips said. “We’re out there talking, and we’re like, ‘Let’s win this for Johnny.’ ”

Phillips hit a two-run homer in the third, and Jay Bruce added a solo drive leading off the fourth to pace the Reds’ offense.

Phillips added an RBI single in the ninth for his third hit and the Reds scored another on a passed ball.

Reds closer Aroldis Chapman gave up a run in the ninth on a wild pitch but struck out Buster Posey with a 100 mph fastball to end it with runners on second and third.

San Francisco’s Matt Cain allowed his first career postseason earned runs after going untouched during the Giants’ improbable World Series season of 2010.

“The pitch to Bruce wasn’t too terrible, but the hanging breaking ball to Phillips is just something that you don’t want to happen in a big-game situation like this,” Cain said.