ST. LOUIS -- Brandon Pfaadt was in complete control on Monday night, until he wasn't.

Pfaadt was unhittable in the early going of the D-backs' 5-3 loss in the first of a three-game series against the Cardinals at Busch Stadium.

He retired the first 12 batters he faced. Even after Nolan Arenado singled to lead off the fifth to break up the perfect game, Pfaadt responded by sandwiching strikeouts of Paul Goldschmidt and Iván Herrera around a groundout to Alec Burleson to complete a dominating five-inning stretch.

Then it fell apart.

"It was a tough one to swallow," Pfaadt said. "You know, I felt great. Threw five innings and went back out there and tried to attack and obviously we didn't do that."

Pfaadt walked the bases loaded to begin the sixth. He struck out Willson Contreras, before a Lars Nootbaar 2-run single ended his night.

"I was trying to make an adjustment in the game and obviously I wasn't finding it," Pfaadt said. "And, you know, that's kind of the funny thing about baseball is you can feel so good and it flips the script pretty quick."

The three-straight walks were surprising, given Pfaadt's reputation as a strike thrower.

"There was a mound visit and there were some things that showed me he was going to be able to get back in the zone and it just didn't happen," D-backs manager Torey Lovullo said. "So, he fills up the strike zone. He's a strike thrower deluxe, and it just got away from him."

Pfaadt's final line was 5 1/3 innings, two runs, two hits, four strikeouts and three walks. It is not unlike the lines from his previous starts this season.

But given the domination of the first five innings, Pfaadt left with a feeling it could have been more.

"Everybody deserved better than that and I think that kind of got the momentum on their side in that moment when I walked bases loaded and I think that that was the focal point of them gaining the momentum to win the ballgame," Pfaadt said.

The D-backs' fortunes turned almost as quickly as Pfaadt's night did.

While Pfaadt was rolling, Arizona had jumped out to a 3-0 lead. Eugenio Suárez drove in two runs as part of a three-hit night for the D-backs.

"I feel like we let the Cardinals hang around, we let them just linger and I sensed that Brandon was in control of this game and we had some opportunities to really break this game open offensively and we didn't," Lovullo said. "It's understandable, but we let a baseball team hang around and when you do that, at this level, they're going to strike back and they did."

Goldschmidt's homer in the sixth off Scott McGough tied the game at 3-all. Nolan Gorman's two-run walk-off home run off Kyle Nelson completed the comeback.

"It might have been a slider that just hung in the middle of the plate and you know that player's a good player. … He's just put a good swing on the ball and that's what happens," Lovullo said of Gorman's homer. "You know, they just out executed us the right times and kind of got to move on."

Pfaadt's abrupt exit changed the script for Lovullo.

"That's the beauty of sports, right?" Lovullo said. "He had it going on and then the next thing you know, it just flipped on its head. So, we were extremely short in the bullpen … and we were trying to grind through some different moments."

But Lovullo wouldn't pin everything on Pfaadt's sudden loss of control.

The D-backs left 10 runners on base. That included a bases-loaded opportunity in the second inning after scoring their first two runs and runners on second and third with an out in the fifth after a Suárez ground-rule double had made it 3-0.

Each time, Cardinals starter Lance Lynn was able to limit the damage by denying Arizona the game-breaking hit.

"We got to just be patient, see ourselves through the count and get into a good spot to where the pitcher has to throw a strike instead of going early and swinging at his pitch," Lovullo said.