SAN DIEGO -- Chris Denorfia was sitting on a fastball.
"He's not going out there trying to trick anybody. He throws 100 miles per hour," Denorfia said of Reds closer Aroldis Chapman. "Even if you're going to try to hit a slider, you've got to be ready for that fastball."
Denorfia drilled the first pitch -- a fastball -- off Chapman for a pinch-hit, two-run homer with no outs in the ninth, rallying the San Diego Padres to a 2-1 victory over the Cincinnati Reds before 24,050 fans Monday night at Petco Park.
Denorfia's game-winning home run traveled an estimated 423 feet over the center-field wall after Chapman walked Yonder Alonso to lead off the ninth.
"I don't know if I can describe what it's like to hit that," said Denorfia, who registered his first walk-off homer. "It's coming in so fast, you don't have time to do anything. I just try to simplify my swing as much as I can and put a barrel on it. (Chapman) provides most of the power.
"It was pretty sweet. That's one of those where if you hit it on the good part of the bat you don't feel any vibration or anything like that. Those are the ones that you know you got good."
It was the fourth blown save in 28 chances for Chapman (3-4) and the fifth home run he's allowed this season.
"It was a fastball like I throw everybody," Chapman said of the pitch to Denorfia. "I just threw a fastball, and he hit a home run. There's nothing I can do about it."
The Reds (59-48), who have lost four in a row, were beaten on a walk-off home run for the second consecutive game. On Sunday, Yasiel Puig launched a solo shot in the Los Angeles Dodgers' 1-0, 11-inning win over Cincinnati.
San Diego (49-58), which won its third straight, earned its fifth walk-off win, but it was the first time this season the Padres won one with a homer.
Reds starter Mike Leake blanked the Padres on four hits in seven innings but didn't factor in the decision. Leake recorded five strikeouts and issued two walks, retiring 12 of the last 13 batters he faced. The only exception in that span was when Everth Cabrera reached on a fifth-inning error.
Reliever Luke Gregerson (6-5) retired the Reds in order in the ninth for the victory.
San Diego starter Sean O'Sullivan allowed a run on five hits and five walks in six innings. He struck out one. O'Sullivan, whose wife is expected to deliver a baby Tuesday, wasn't pleased with his overall performance, but he was glad he gave up just one run.
"I'd say tonight's outing was one of my most frustrating," said O'Sullivan, who is winless as a starter despite a 2.60 ERA. "I didn't feel like I had a very good feel for my fastball. I was choking it off a lot. But I thought I made some key pitches in situations that kept us in the game and gave us a chance to have that awesome finish."
The matchup between O'Sullivan and Leake featured two products of San Diego-area high schools. Leake played at Fallbrook High, while O'Sullivan prepped at Valhalla High in nearby El Cajon. The two played against each other in travel ball leagues growing up but had never faced off against each other in the majors. O'Sullivan said the two remain friends and spoke before the game.
The Reds broke up a scoreless contest in the fifth when Derrick Robinson, who led off the inning with a triple to right-center, scored on Cesar Izturis' grounder to first base.
It was the Reds' first run in 22 innings. They hadn't scored since the second inning of Saturday night's loss to the Dodgers.
NOTES: Reds 2B Brandon Phillips was out of the starting lineup. Manager Dusty Baker said he wanted to give the All-Star a rest. ... Cincinnati CF Shin-Soo Choo missed the game with a twisted left ankle. Choo was hurt sliding into second base during Sunday's game against the Dodgers. His status is day-to-day. ... Padres LF Carlos Quentin didn't play Monday because of a "slight tweak of the knee," manager Bud Black said. Quentin, who has 20 RBIs in July, tied for third best in the National League entering play Monday, hurt his left knee Sunday during an at-bat at Arizona. ... Reds RHP Jonathan Broxton is scheduled to begin a rehab assignment at Triple-A Louisville on Tuesday. Broxton has been on the disabled list since June 15 with a strained right elbow.