PHOENIX -- The suddenly slumping Arizona Diamondbacks never saw it coming from Reds rookie Luis Castillo.

Castillo was overpowering at times while pitching 6 2/3 shutout innings for his first career victory and Joey Votto hit a three-run homer in the first inning to carry the Cincinnati Reds past the Arizona Diamondbacks 7-0 on Saturday night.

Castillo (1-1) was in command throughout, touching 100 mph with his fastball several times during his fourth career start. He struck out eight, walked one and gave up only one hit until the seventh inning, when he was lifted after allowing two singles.

"You couldn't ask for more than that -- that was a great performance," Reds manager Bryan Price said. "He had a nice mix, he really did. (He) was able to bring in the changeup and the breaking ball but threw the fastball with confidence, sitting there at 97 to 100, and blending in the others when needed."

Votto knew how well Castillo was pitching from the reaction he saw from the Diamondbacks' frustrated hitters: "Guys are getting awkward swings and don't seem comfortable up there."

"I was very impressed with his composure and his presence," Price said of the 24-year-old Castillo. "More than anything, he exudes confidence out there. From everything I pick up when I go to the mound, the infielders love playing behind this guy. They love seeing him get after it, pound the zone. He's confident and he trusts his stuff, and the team loves playing behind him."

Five of the Reds' seven hits went for extra bases. Scooter Gennett and Tucker Barnhart also homered, and Barnhart doubled and scored on Billy Hamilton's single in the fourth inning.

Hamilton reached base four times with two walks and both Reds' singles and stole second three times.

The right-handed Castillo, acquired from Miami last winter in the Dan Straily trade, was never in trouble until Jake Lamb and Ketel Marte singled two batters apart in the seventh. Castillo then left after throwing 113 pitches, with Wandy Peralta coming on to get pinch hitter A.J. Pollock to pop out with two on and two out.

"You're always wondering as a professional pitcher what is going to be your first win and today was it, and I'm real happy," Castillo said through an interpreter. "I tried to get ahead with my fastball and I had really good feel for my changeup and it worked out really well for me."

Peralta also pitched the eighth before Ariel Hernandez worked the ninth to finish off the three-hit shutout.

Castillo faced a lineup that didn't include two of Arizona's best hitters, David Peralta and Pollock, though Pollock entered later in the game after pinch hitting. Manager Torey Lovullo defended his lineup, saying he needs to keep players rested for the August and September stretch drive.

"Whether we win or lose, these guys are always prepared," Lovullo said.

Diamondbacks starter Taijuan Walker (6-4) gave up five runs and five hits in five innings and surrendered multiple homers in a game for the first time this season.

Arizona lost for the fourth time in five games and seventh in 10 games to fall 6 1/2 games behind Los Angeles in the National League West.

The Reds needed only three batters to take a 3-0 lead against Walker, who allowed only one run in seven innings in his previous start on July 1 against Colorado.

Hamilton walked and immediately stole second -- a pattern he was to repeat -- before All-Star shortstop Zack Cozart scored him with a triple to right-center field.

Votto, en route to his fifth All-Star Game but first since 2013, then hit an opposite-field homer to left field on an 0-1 fastball that Walker left up in the strike zone. Votto's homer was his National League-leading 26th, three shy of the 29 he hit last season.

The Reds, who came in 18-34 in their previous 52 games, made it 4-0 before the Diamondbacks got their first hit against Castillo. Barnhart doubled with one out in the fourth and scored on Hamilton's two-out single to left. Hamilton then stole second for his 37th of the season and third in as many innings.

Hamilton also walked and stole second in the second but was stranded when Cozart grounded out.

Paul Goldschmidt singled up the middle leading off the fourth for Arizona's first hit, but Castillo promptly retired the next three batters.

Gennett hit the Reds' second homer of the game and the seventh this season off Walker with one out in the fifth, a solo shot that was his 15th and his third during a seven-game hitting streak.

NOTES: Reds RHP Luis Castillo has struck out 25 in 18 innings in his last three starts. ... Diamondbacks All-Star pitchers Zack Greinke (11-4) and Robbie Ray (8-4) won't start in the three-game series in Atlanta that begins Friday, according to manager Torey Lovullo, even if neither appears in the game Tuesday. ... Greinke started the 2015 All-Star Game while with the Dodgers. ... Reds 1B Joey Votto has homered in each of the first two games of the series, driving in six runs. ... Arizona RHP J.J. Hoover (right shoulder inflammation) threw 23 pitches during a simulated game. ... CF Billy Hamilton's three stolen bases matched the major league season high, also reached by four others. Hamilton also had three steals versus Pittsburgh on May 1. ... Reds C Tucker Barnhart homered in the ninth inning off LHP Jorge De La Rosa. ... Arizona was shut out for the fourth time this season and the first at home.