When Upton lifted Greinke's full-count slider into the seats in left-center and closer Craig Kimbrel got the final three outs to reward starter Ian Kennedy and setup man Brandon Maurer, the Padres had a 2-1 victory that lifted them to .500 and within five games of the front-running Dodgers in the National League West.
This is what Padres general manager A.J. Preller had in mind when he acquired Upton from the Braves over the winter to pair him up with Matt Kemp in the heart of a revitalized lineup.
"That's what makes the game fun," Upton said. "I think if you ask most guys, they enjoy those situations. They want to be up there."
From where he was standing, breathlessly taking it all in, Black couldn't have been more thrilled to see his left fielder come through in one of those moments great players make their own.
"You look at Greinke, and you look at Upton," Black said. "You're talking about All-Star performers. Those guys are thriving in those moments, both ways: two high-caliber players in a very intense situation.
"Greinke throws a 3-1 slider, a great pitch, and Justin swings and misses. Then he throws a 3-2 slider, and Justin gets it. That's what makes the game great -- when two very good ... two great players are matched."
In front of a festive sellout crowd numbering 43,525, the Padres struck first, in the first. Will Venable doubled and scored with two down on a slow roller by Kemp that Greinke -- the 2014 NL Gold Glove Award-winning pitcher -- chucked wildly past first for just the sixth error of his career.
Yasmani Grandal, the former Padres catcher, crushed a tying homer to right in the third. The Dodgers didn't get another man in scoring position the rest of the night as Greinke used his guile to escape jams in the fifth and seventh innings.
Dodgers manager Don Mattingly let Greinke, a 2014 NL Silver Slugger Award winner, bat in the seventh, and he flied out. Greinke's bid to end a winless drought now spanning seven starts ended with Upton's stroke of fortune.
A .391 career hitter against Greinke in 23 at-bats coming into the game, Upton had a hit taken away in the sixth on a sprawling stab by shortstop Jimmy Rollins for a force at second. In his other two at-bats, Upton had grounded out and struck out.
"He'd kind of been around the edges, down around the knees, all day," Upton said. "I had to kind of wait him out and hope to get a mistake."
Greinke's slider got just enough of the hitting zone for a highly evolved hitter to launch it. Mistakes at this level can come by fractions of inches in location.
"I see him pretty decent," Upton said. "He's a good pitcher. He got the best of me for three at-bats. He had his slider going; he was throwing it for strikes. Same thing with his heater. He had everything going. He mixed it up and down."
Greinke is now 5-2 with a 1.95 ERA. In all likelihood, he'll join Upton in Cincinnati for the July 13 All-Star Game, representing the NL. Until then, they'll be busy trying to take care of business in a stacked division very much up for grabs, with the defending World Series-champion Giants always a threat to get hot and take flight.
The Padres will need more efforts such as this one to hang with the other two California clubs, but the talent clearly is there to make it happen.
"Granted, we're still trying to break through and climb over .500," Black said. "We've had a number of wins in the early season that were huge -- and this was another one."
Kennedy, whose season began with a hamstring strain, showed off the kind of stuff that can make him a force in a potentially dynamic rotation. The bullpen is deep, with no better finisher in the game than Kimbrel, the former Brave.
The ninth inning opened with Yasiel Puig winning a stirring eight-pitch confrontation with the Padres' closer by banging a 97 mph heater through the middle for a single.
Up came Adrian Gonzalez, a .326 hitter against his former club who had been ejected on Friday night in a dispute over balls and strikes. Kimbrel got him to tap a breaking ball back to the mound for a textbook double play nicely turned by shortstop Alexi Amarista.
When Howie Kendrick waved at a two-strike breaking pitch, Petco Park erupted.
Twenty-four hours after the Dodgers had seized the series opener with a late rally, those familiar "Beat L.A.!" chants were answered with one Justin Upton swing.