Spencer Steer also had a two-run homer and rookie Andrew Abbott struck out a career-high 12 in 7 2/3 innings for the surprising young Reds, who remained tied with Milwaukee atop the NL Central.
“That’s what a team is all about. It’s not the same guy every day," Cincinnati manager David Bell said. “It is very special.”
Ha-Seong Kim and Fernando Tatis Jr. hit back-to-back homers in the eighth to tie it 2-all, but the disappointing Padres lost for the 10th time in 13 games.
“We go back home, hopefully things change,” shortstop Xander Bogaerts said. “There’s been a lot of tough losses lately, man. It’s been really tough.”
Stephenson batted for slumping Joey Votto and launched the first pitch from Nick Martinez into the visitors' bullpen in right field for his fifth career pinch-hit homer.
“In that situation, you’ve got to be aggressive,” Stephenson said. "I happened to get a pitch I could handle and I could drive and it was good to see it go over the fence. It was huge and a really cool experience.”
San Diego trimmed it to 4-3 in the ninth when Jake Cronenworth's two-out double scored Gary Sánchez, but Alexis Díaz retired pinch-hitter Rougned Odor on a line drive to second base for his 23rd save.
After allowing two singles in the first, Abbott retired the next 15 batters with eight strikeouts. The left-hander permitted one run and four hits with one walk, setting down 21 of his final 25 batters.
“It was a steady mix. All four of my pitches were in the zone early, then diving out late," Abbott said. "I felt really good from the get-go. I just had to go out and execute and hit my spots.”
Abbott has a 1.21 ERA after six career starts. The Reds are 6-0 in those games.
“He believes in himself,” Bell said. "He respects the league. He respects his opponents. But he also knows what he’s capable of. He’s not going to back down. It's a mindset.”
Kim’s homer ended Abbott’s day. Tatis tied it at 2 with a drive off winning pitcher Lucas Sims (2-1).
Tom Cosgrove (1-1) took the loss.
Adrian Morejon tossed three scoreless innings for the Padres in place of Yu Darvish, who hasn’t pitched since June 21 due to illness.
Steer launched his 14th home run of the season, a two-run shot into the upper deck in left field off Tim Hill to put the Reds ahead 2-0 in the fourth.
Steer is the first Reds rookie since 1953 to reach 50 RBIs before the All-Star break.
Cincinnati has won seven of its last eight series.
For the Padres, it was another difficult defeat.
“We hit two home runs, and momentum is back in our dugout,” manager Bob Melvin said. “They had one more big swing than us. Two swings, all their runs.”
Votto homered three times in his first four games after returning from shoulder surgery. But after going 0 for 2 on Sunday, the first baseman is hitless in his past 20 at-bats, his longest skid since going 22 straight in August 2022.
“I know he wants to produce every game, and we're so accustomed to seeing that,” Bell said. “It's still an adjustment. I know he physically feels great. Sometimes it's just a matter of time.”
Reds right-hander Derek Law (right elbow strain) was reinstated from the 15-day injured list.
Padres: Had not announced a starter for Monday’s series opener against the Los Angeles Angels.
Reds: RHP Luke Weaver (1-2, 6.96 ERA) starts Monday at last-place Washington.
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