Prado leaned across the plate and knocked a slider the opposite way to right for a two-run single, as the D-backs fell, 6-5, to the Braves in front of 30,657 at Turner Field.
"I was mentally throwing sinkers down hard," Qualls said. "I was forcing him to hit a ground ball. It was a slider on the black down and away, and it was probably the weakest swing he took all night and he wins the game. Obviously I have to tip my hat, because he made contact with it and kept it fair, but it's just super frustrating when mentally I'm on the mound knowing he's going to hit a double-play ball.
"There's no doubt in my mind we're going to win that game."
The loss was the seventh in a row for the last-place D-backs, while the Braves won their fourth straight.
"I'm at a loss of words for this one, because this one is pretty tough," D-backs manager A.J. Hinch said.
The way Ian Kennedy was pitching early on, it didn't look like the D-backs were going to have to rely on their bullpen.
The right-hander allowed just four hits while keeping the Braves off the board through the first six innings.
"Kennedy was very good," Braves manager Bobby Cox said. "He can spot the ball. He's like a Greg Maddux out there with his control. He was right on the black all night long. He's not a big strikeout guy, but God, he doesn't give you anything to hit."
Meanwhile, the Arizona offense, which had struggled of late, managed to score four times off Atlanta starter Kenshin Kawakami.
Brian McCann led off the bottom of the seventh with a solo homer, and after a strikeout, the Braves put together back-to-back singles. At that point, Hinch felt Kennedy was done.
"He got a little tired at the end, but he did his job," Hinch said.
Said Kennedy, "For me it was just executing. I didn't feel tired."
In came Juan Gutierrez to face Nate McLouth with runners on the corners. While the right-hander had struggled more than any other reliever of late, Hinch liked the matchup because he thought Gutierrez's mid-90s velocity would be difficult for McLouth to catch up to. McLouth came into the game hitting just .167, and Kennedy had struck him out in his previous two at-bats.
Gutierrez, though, fell behind in the count, and when it got to 3-2, McLouth was able to drive a 95-mph fastball into the right-field seats to tie the game at 4.
"He fell behind and then he got predictable in a fastball count," Hinch said. "The one thing, when [McLouth] knows a guy has to throw a fastball, he's going to cheat a little bit to get to it, and he did. The only thing that could hurt us in that situation was a home run."
Still, the D-backs had life in this one, and they responded by pushing across a run in the ninth when Justin Upton singled home Conor Jackson to give Arizona a 5-4 lead.
Eric Hinske led off the ninth with a single to center on Qualls' first pitch.
"Obviously, get ahead," Qualls said of his approach with Hinske. "And obviously his approach against me was just to ambush me and swing at the first pitch. He didn't even hit it that hard, he just got a single. I automatically thought, 'Let's get a double play.'"
Omar Infante then bunted, but Qualls was unable to field it cleanly when the ball landed in front of him with some backspin, and the Braves had runners at first and second with no one out.
"I called [catcher Chris] Snyder off," Qualls said. "I went to go grab it and it wasn't there."
McLouth then laid down a successful sacrifice bunt, and the D-backs walked Melky Cabrera to load the bases for Prado.
"To get a base hit going to the opposite field is pretty good hitting," Hinch said of Prado. "Everything that could go wrong did go wrong in that inning. It's been kind of a pattern."