Segura went deep against Chris Heston for his third spring homer. He was acquired in a January trade with Milwaukee.
Rickie Weeks also homered for Arizona, and Chris Owings had two hits. Kyle Drabek pitched four innings of one-run ball.
''The guys had good at bats all night,'' Diamondbacks manager Chip Hale said. ''Everybody threw in a hit here or there.''
Heston allowed four runs and six hits in four innings. He struck out five and walked two.
''Heston gives us fits with changing speeds,'' Hale said. ''Guys had to adjust and Segura got us going.''
Brandon Belt and Matt Duffy each drove in a run for the Giants. Duffy finished with two hits.
Diamondbacks: Drabek came over from minor league camp to pitch instead of LHP Robbie Ray and threw three scoreless innings and allowed one run in the fourth. Ray got his work in elsewhere.
''He's kind of reinvented himself, after two Tommy John surgeries, as a sinkerball pitcher,'' Hale said. ''You saw all the ground balls he got that turned into double plays. We wanted to see him again. He looks really good.''
Giants: Heston pitched a no-hitter against the New York Mets last year but will likely open the season in long relief.
''I haven't heard anything specifically,'' Heston said. ''I'll just keep pitching. I made a few relief appearances when I was first called up and I learned I can get ready way quicker than I thought I could.''
Heston thinks pitching out of the bullpen can make him that much more valuable to the organization.
''It makes yourself versatile learning new roles,'' he said. ''Once roles get defined you can get into a routine and that will be huge.''
Heston said he needs to stay away from the big inning. He gave up three runs in the fourth against the Diamondbacks.
''I need to keep the ball down in the zone and let the sinker work,'' Heston said.
Diamondbacks: OF A.J. Pollock (right elbow soreness) began playing in minor league games recently. He's been out since March 8.
Giants: RHP Matt Cain threw 90 pitches in a minor league game Monday and should be ready for the regular season. ... C Andrew Susac (sore right wrist) caught Cain and faced live pitching for the first time in a while.
Diamondbacks: Arizona has yet to announce a starter for Tuesday's game against LHP Chris Rusin and the Colorado Rockies.
Giants: RHP Clayton Blackburn starts for the Giants on Tuesday night against the Kansas City Royals and RHP Yordano Ventura in Surprise. ... LHP Madison Bumgarner will pitch in a minor league game.