''It's bases loaded, nobody out, we need one run, there's obvious pressure there,'' said Calhoun, who's from nearby Tempe and went to Arizona State. ''I just tried not put too much on it, just go out and have fun in the moment, let the results just take care of themselves.''
Arizona trailed 4-3 after Alex Bregman hit a two-run homer off Stefan Crichton in the eighth inning. Junior Guerra (1-0) worked around an error in the ninth inning and the Diamondbacks loaded the bases against Ryan Pressly (0-1) in the bottom half.
Calhoun laid off a couple of pitches after falling into a hole and sent the Diamondbacks charging out of the dugout with a line drive to the corner in right. The Diamondbacks weren't sure how to celebrate while remaining socially distant, but they can work on that after finally winning a series.
''I think we're going to have to get better at that,'' Diamondbacks manager Torey Lovullo said. ''It's probably the most unusual feeling I've had after a walkoff win.''
Arizona starter Zac Gallen tied an NL record set by Montreal's Steve Rogers (1973-74) by allowing three earned runs or less in his first 18 career starts. Gallen allowed two runs and six hits and struck out six in six innings.
Jose Altuve hit a solo homer off the Arizona right-hander in the fourth inning and had his first three-hit game, finishing with a double and three runs. The Astros were in good shape to win the series after Bregman's homer, but Pressly, who's been battling a blister issue, couldn't get an out.
''He hadn't had much work and he was confident to pitch and get some work,'' Astros manager Dusty Baker said. ''We had it all lined up, the perfect situation, but the Diamondbacks messed up our plans.''
Astros starter Brandon Bielak was sharp in his first big league start after two relief appearances, allowing two hits in five innings.
The Diamondbacks took advantage after the rookie right-hander left.
Christian Walker hit a run-scoring double off Cy Sneed in the sixth and David Peralta tied it with a single through the right side off Blake Taylor. Andy Young followed with a groundout for his first big league RBI to put Arizona up 3-2.
Gallen allowed three earned runs in 10 innings his first two starts but did not factor in the decision in either. He struck out nine in six innings his last start and opened Thursday's game by striking out the side.
Gallen gave up a run-scoring single to Yuli Gurriel that hit the third base bag in the fourth inning and Altuve launched a solo homer off him in the sixth to put Houston up 2-0.
Altuve, the 2017 AL MVP, finished 3 for 4 Thursday night and had a pair of homers in the series.
''Before today's game, I felt sometimes a little nervous, a little late,'' he said. ''Hopefully after today's game I can keep it going and keep helping my team.''
Young got his first big league start as Arizona's designated hitter and though he went 0 for 2, hit the ball hard and got his first career RBI.
''Unbelievable at bats. He's got heavy hands. He's a thumper,'' Lovullo said. ''When he gets hit pitch, you can see what he does. He put some good swings on some balls.''
Arizona designated INF Ildemaro Vargas for assignment and optioned RHP Yoan Lopez to its alternate training site to get its active roster down to 28.
The Astros got down to the 28-player limit by optioning RHP Bryan Abreu and INF Jack Mayfield to their alternate training site.
Astros: Reigning AL rookie of the year Yordan Alvarez took batting practice and did some running at the team's alternate training site. He arrived at summer camp late for an undisclosed reason. ... RHP Jose Urquidy (undisclosed) has begun throwing off a mound.
Astros: RHP Zack Greinke (0-0, 5.00 ERA) will start against Oakland on Friday after getting an extra day's rest.
Diamondbacks: RHP Luke Weaver (0-2, 14.73) looks to get his shaky start to the 2020 season on track in Arizona's series opener against San Diego on Friday.
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