And it might have cost the Cubs the game.
The go-ahead run was waved home automatically in the sixth inning after Rizzo came down with the ball in his glove for the out. By rule, if a defensive player takes the ball out of play, each runner is awarded a base.
The only run Collmenter allowed was Rizzo's 23rd home run of the season, matching his total for all of last year.
Collmenter (8-5) gave up just three other hits.
"I was able to command the fastball both sides of the plate," he said. "With a lot of lefties in the lineup I was able to pitch in really well, got a lot of popups and ground balls and the defense made some great plays."
David Peralta had a run-scoring double to tie it, then came home from third on Rizzo's camera well plunge.
Rizzo said it was "just instinct" that he lunged for the ball.
"I didn't think I would go in (the camera well)," he said. "But it was a little farther then I thought."
The Diamondbacks earned their second sweep of the season, first at home.
The Cubs have lost 11 of 13 since Jeff Samardzija was traded to Oakland on July 5.
Jake Arrieta (5-2), who had a stomach virus the past few days, allowed three runs and four hits in 6 2/3 innings for the Cubs.
It was a tough series, tough game today," he said. "The challenge really for me was what I had to deal with the last few days, trying to hold fluids down. My body was weak not being able to eat any solid food.
Addison Reed pitched a perfect ninth for his 23rd save in 28 tries.
The Diamondbacks are 3-0 after the All-Star break for the second time in franchise history. The other was in 2002.
The game was scoreless through five innings, then Rizzo lashed the first pitch he saw in the sixth into the seats in the right corner, moving into a tie for the NL homer lead with Miami's Giancarlo Stanton. Rizzo homered twice in the series opener.
Arizona scored twice in the sixth, the first on consecutive doubles by Ender Inciarte and Peralta. Peralta scored from third when Rizzo caught Aaron Hill's high pop foul as he fell over the bar protecting the camera well near the Cubs dugout.
In hindsight, Rizzo might have been better off just letting the ball drop foul.
But, he said, "I don't think there is any Cubs fan that wanted me to miss it or any Diamondbacks fan that wanted me to catch it."
The Diamondbacks won a challenge in the seventh when Mark Trumbo was called out stealing second. After a review, the call was reversed. Trumbo scored when Peralta singled.
Arizona came from behind in all three games. ... Chicago's Chris Coghlan has hit safely in 16 of his past 17 games. ... The Cubs have Monday off, then open a 10-game homestand with a three-game series against San Diego. The Cubs' RH Kyle Hendricks (0-0, 6.00 ERA) makes his second big-league start in Tuesday's opener. The Padres go with LH Eric Stults. Arizona opens a three-game interleague series with Detroit. LH Vidal Nuno starts for Arizona, RH Rick Porcello (12-5, 3.39) for the Tigers.