Three innings later, Rizzo squared up and drove a triple off the fence in left-center for his first hit, then scored his first run.
It was a solid start for Rizzo, who helped the San Diego Padres beat the Washington Nationals 7-3 on Thursday night. The seven runs tied the season high at home this season for the Padres, who have struggled at Petco Park.
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"He looked as though he belonged on the field," manager Bud Black said. "From the time the game started, he looked poised, looked composed, had some good at-bats, knocked the heck out of one.
"Just the overall swings, just the at-bats, he looked in control of his game."
The 21-year-old Rizzo, one of three prospects obtained from Boston for All-Star Adrian Gonzalez in December, was called up earlier in the day from Triple-A Tucson. He started at first base, batted seventh and finished 1 for 2 with two walks.
"It was awesome. It was a dream come true," Rizzo said. "I stepped in the box and just felt so comfortable. It was one of those days where you're picking up the pitches and seeing everything great. Great day.
"I just tried to keep my composure, tried not to let anything go absolutely, terribly wrong."
He did tumble part way over the low wall along the first-base seats going hard after a foul ball that was probably five rows deep.
"They told me to relax, if you want to stay here," Rizzo said.
Chase Headley hit a two-run homer for the Padres.
Hernandez (3-8) tied for the major league lead with his eighth loss. He is 0-6 in his last eight starts overall and 0-6 in his eight road starts. He's also 0-6 in eight career starts at Petco Park. The six straight losing decisions tie the longest streak of his career.
Hernandez did make a terrific play by catching Chris Denorfia's screaming line drive at his face in the third. Hernandez sprawled on his backside on the mound after making the catch, taking off his glove and looking at his hand.
Hernandez allowed six runs and nine hits in 5 1/3 innings, struck out four and walked one.
San Diego's Aaron Harang (7-2) went six innings, holding Washington two runs and six hits while striking out four and walking three. The seven wins are his most since 2007 with Cincinnati.
Rizzo tripled off the wall leading off the fifth and scored with two outs when Denorfia beat out an infield single. Rizzo had one triple during his five seasons in the minors, including this year with Tucson. Rizzo earned the promotion by batting .365 with a .715 slugging percentage, .444 on-base percentage, 16 home runs and 63 RBI at Tucson.
Rizzo thought it might have been gone.
"I was talking to it. I was definitely telling it to get out. I hit it well but I didn't know if I had enough height."
In his first at-bat, Rizzo was punched out by umpire John Hirschbeck on a strike that appeared to be inside.
Headley ended the longest homerless streak in his career with an opposite-field shot to left-center in the first inning. It was his second of the season and first since April 2, a span of 194 at-bats and 229 plate appearances.
The Padres scored three runs in the sixth, when they chased Hernandez. Brad Hawpe had an RBI single, pinch-hitter Kyle Phillips drew a bases-loaded walk and pinch-hitter Logan Forsythe hit a sacrifice fly.
Harang threw 24 pitches in the first inning, allowing a triple and a walk but no runs. The Nationals loaded the bases with no outs in the second and got only one run, on a sacrifice fly by Alex Cora. Hernandez's sacrifice was the second out and Rick Ankiel fouled out. Alex Cora had a sacrifice fly in the second and Michael Morse an RBI single in the sixth.
"It was not a good game," Nationals manager Jim Riggleman said. "The effort and intensity were there but we had a couple of things happen. It was not a pretty game. I don't know how to say it."
Nationals pinch-hitter Wilson Ramos homered with two outs in the ninth, his fifth. It was his first career pinch homer.
- Will Venable, recalled earlier in the day from Tucson, made a leaping catch at the fence in center to rob Danny Espinosa of a home run to end the eighth.
- In his past five starts, Harang is 2-0 with a 1.87 ERA. He has worked six innings or more in 11 of 13 starts.
- The Padres are 29-11 against the Nationals since 2005, the year they moved to Washington.