In the bottom of the second inning, Rizzo swung on the first pitch from Nationals starter John Lannan, a 92-mph fastball low and inside.
The result? A 378-foot bomb over the right-field wall and Rizzo's first Major League home run.
"When I hit it, yeah, I know I got it pretty good, and it got over," Rizzo said. "So I was happy. Very happy."
Rizzo has impressed the Padres with his patience and poise at the plate, and the Nationals have also come away with the same feeling after his first three games.
"It was a four-seam [fastball]," Lannan said. "I was trying to throw it away. He put a good swing on it. He hit it. He has impressed us with the way he has handled each at-bat in this series. He is going to be really good."
Rizzo rounded the bases and got a high-five from the Padres' next batter, Nick Hundley, on his way back to the dugout. Rizzo had 16 home runs in 52 games with Triple-A Tucson before being called up by the Padres on Thursday.
After his third game, Rizzo is 3-for-7 with four walks, three strikeouts and a hit-by-pitch. His three hits are a triple off the left-center-field wall, a home run and a double down the right-field line. Though his sample size is small, Rizzo is batting .429 with a .667 on-base percentage.
"It was good to see," Padres manager Bud Black said. "He hit it to one of the biggest parts of the park, so it shows his power. And then the double down the line ... I mean, that's the thing we saw in Spring Training: all-field hitting, all-field power ... and there's pull power as well."
The home run moved the Padres within one run of the Nationals, but it was the only run San Diego scored in its 2-1 loss on Saturday.
After the game, the 21-year-old lefty was able to retrieve his first home run ball. In exchange, Rizzo offered an autographed bat and some signed baseballs.
"It's more important for my family [to get the ball back]," Rizzo said. "I'm going to probably give it to my parents. I'm really more happy for them than I am for myself."