SAN FRANCISCO (AP) Bruce Bochy tried Madison Bumgarner and another starting pitcher, Mike Leake, as pinch hitters.
San Francisco rallied back once and then again, and still fell short.
Todd Frazier hit his 34th home run leading off the 10th to make up for a costly throwing error an inning earlier, and the Cincinnati Reds held on for a wild 9-8 victory against the Giants on Tuesday night that snapped San Francisco's four-game winning streak.
''Our season's pretty much on the line every time we go out there right now,'' Bumgarner said.
Frazier connected against Sergio Romo (0-5).
Aroldis Chapman (4-4) recorded five outs for the win despite his third blown save. Jumbo Diaz earned his first career save, finishing the 4-hour, 2-minute game.
Buster Posey hit a tying RBI single in the ninth as the Giants evened it up following Frazier's wild throw to first after the third baseman made a diving stop on Matt Duffy's infield single, allowing a run to score.
''I was a little more emotional just because I made that throwing error that was costly,'' Frazier said. ''Whenever you pick yourself up I think you're going to be a little more emotional.''
Defending World Series champion San Francisco lost for the first time in five games on this key nine-game homestand. The Giants began the night 7 1/2 games behind the NL West-leading Los Angeles Dodgers and 6 1/2 games back of the Cubs for the second wild card.
Four starting pitchers between the teams had plate appearances.
Bumgarner walked as a pinch hitter against Chapman in the eighth to load the bases for Ehire Adrianza, who was hit by a pitch to force home a run. Chapman was checked on by a trainer with one out in the ninth but stayed in the game.
''You want me to tell you he throws hard? That's pretty clear, he throws hard,'' Bumgarner said. ''It was 102 or whatever it was. I'm trying to hit the ball just the same.''
In the seventh, San Francisco challenged when Skip Schumaker appeared to score when Jason Bourgeois reached on an error. The review of 2 minutes, 4 seconds was overturned and showed that right fielder Marlon Byrd made a perfect throw to Posey in time for the catcher to tag out a sliding Schumaker.
Jay Bruce followed with an RBI single to pull the Reds to 5-4 before Brandon Phillips' RBI single against Hunter Strickland evened the game. Frazier was hit by a pitch to load the bases for Ivan De Jesus, who drew a nine-pitch walk to put the Reds ahead. He added to his earlier two-run triple.
Adam Duvall added a two-run single to cap the crazy seventh, when the Reds scored five runs despite having two runners thrown out at the plate.
''Entertaining but a tough loss. The boys battled hard,'' Bochy said. ''The seventh inning killed us. A couple errors, walks, hit batsmen, you're asking for trouble then. ... We have not beaten ourselves too often.''
Pinch-hitter Jarrett Parker hit his first career home run in the fifth to put the Giants ahead and Posey hit a tying three-run drive in the fourth. Brandon Belt added an RBI double in the sixth to make it 5-3 but the Giants' typically reliable bullpen couldn't hold it.
Chris Heston struck out eight over five innings but saw his winless stretch reach eight starts as he is 0-5 since beating Texas on Aug. 1. Parker pinch hit in Heston's spot and connected with a drive to right-center off Ryan Mattheus in just his sixth career big league game in 18 days of service time.
Belt smacked his head against shortstop De Jesus' left leg trying to get back to second as he was picked off after his sixth-inning double. Belt walked off gingerly alongside athletic trainer Dave Groeschner and the first baseman returned for defense in the top of the seventh.
Reds: CF Billy Hamilton is day to day after he left Monday's game in the fifth inning with a sore shoulder. Hamilton just came off the DL on Sept. 8 after missing 21 games with a right shoulder injury. ... The Reds plan to shuffle their rookie rotation around the time of next week's series at St. Louis.
Giants: SS Brandon Crawford, who missed his sixth straight game with tightness in his calf and left side, is unlikely to play until Friday against Arizona. ... RHP Tim Hudson is expected to make his next start Sunday after his hip flared up and forced him out early of Monday's outing. ... Bochy's hope is OF Hunter Pence (strained left oblique) will be ready to return for the final homestand beginning Sept. 28. ''I can't wait to play again,'' Pence said.
Reds: RHP Michael Lorenzen (4-8) makes his 23rd major league start and second against San Francisco looking to win his second straight decision.
Giants: RHP Jake Peavy (6-6) tries to win his third straight start and fourth consecutive decision when he pitches the series finale.