Upton, 21, had his first career four-hit game in a 2-1, 10-inning loss to Cincinnati on Sunday, when his homer to straight-away centerfield was measured at 434 feet. His Monday homer to the same general area was 424 feet.
Upton hit third Monday for only the third time this season as the Diamondbacks shook up a lineup that had scored only 20 runs in the previous nine games, losing eight.
"It looks like he's swinging a lot better," manager Bob Melvin said. "He's staying on the ball. He's hitting the ball back up the middle, and when he's going good, that's what he does.
"He's the one we have right now who is hitting with a lot of confidence. He feels good."
Upton had a bunt single as a pinch hitter Sept. 10 that seemed to spark a hot streak. After going 0-for-2 with an intentional walk Friday, Upton was 2-for-4 with two doubles and another intentional walk Saturday.
DIAMONDBACKS 3, GIANTS 1: OF Adam Dunn hit a two-out two-run home run in the last of the eighth inning to break a tie at 1-1 as the Diamondbacks won for the second time in 10 games to remain 4-1/2 games behind Los Angeles in the NL West.
OF Justin Upton also homered, and LHP Doug Davis gave up only one unearned run in seven innings while not walking a batter. RHP Chad Qualls got his fourth save of the season, his first since officially being named closer Saturday.