PITTSBURGH -- After six innings, the Pirates were chasing history. When the seventh was over, they were chasing the Cardinals.

Mitch Keller put the Pirates nine outs away from a no-hitter on Saturday night, throttling the Cardinals in only his second start back from a strained left oblique that left him sidelined for six weeks. But Pittsburgh’s bullpen gave up the no-hitter and a four-run lead, allowing five runs in a disastrous seventh inning to waste a start that showcased Keller’s top-of-the-rotation potential.

The Pirates’ 5-4 loss at PNC Park was their 11th in the last 12 games and their 37th in 52 games this season. For all the ways they’ve been defeated this season, this one stood out as uniquely unfortunate.

“It wasn’t easy to watch,” manager Derek Shelton said. “It looked really quick. It looked like we went from 4-0 to 5-4 really quick. We didn’t execute. If we don’t execute, we’re going to put ourselves in those positions.”

• Box score

Shelton was understandably cautious with Keller, prioritizing the former top prospect’s short- and long-term health by adhering to his prescribed pitch count. Keller exited after 65 pitches Monday in his return from the 10-day injured list, so his limit on Saturday was somewhere around the 84 pitches he wound up throwing. Keller hadn’t pitched more than five innings all year, so he didn't have a chance to finish the no-hitter himself.

For those reasons, the sixth inning was bound to be his last no matter what. The Pirates even had Geoff Hartlieb warming up in the bullpen in case Keller ran into trouble during the sixth.

“It was a hard-and-fast cap. It was the first time he had been to six ups,” Shelton said. “Mitch Keller is way too important to the Pittsburgh Pirates. He wasn’t getting through nine. … For him to go seven innings of a no-hitter, no, it’s not worth it.”

Indeed, Keller is an important part of the Pirates’ plan to build a more competitive team in the future. He’s their top young pitcher, and Saturday’s start -- the best of his career -- showed exactly the kind of progress the club hoped to see this season.

“Yeah, it’s huge,” Keller said. “I’ve got one more start to build on ... this outing. I don’t know how much I can build on that, but just continuing with that feeling and just everything I’ve worked on through the rehab process.”

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Keller did not look like himself in his first two outings of this season, when his fastball registered in the low 90s. But he fixed his delivery during his stint on the IL, and his high-octane stuff returned in Cincinnati. It all came together against the Cardinals as he struck out six while walking two batters and hitting another.

The 24-year-old right-hander’s fastball averaged 94.9 mph and touched 97 mph, and it played well up in the strike zone. Like so many other Pirates pitching prospects, Keller was instructed in the Minors to pound the bottom of the strike zone with his fastball to produce ground balls. The new staff in Pittsburgh has helped change that, pairing high fastballs with Keller’s hard-breaking slider and swing-and-miss curveball.

“It was awesome. It reminds me of just playing with him in the Minor Leagues, just seeing him go up there and shove,” said third baseman Ke’Bryan Hayes, who hit his third MLB homer in the first inning. “I wish he could have kept going, but he wasn’t able to.”

When Keller walked into the Pirates’ dugout after a perfect sixth inning, he was greeted with hugs and handshakes. There was no debate with Shelton. Keller only recalled a conversation they had after his last start in Cincinnati, where the pitcher told his manager that something clicked in the third inning. So when Keller came out, he told Shelton, “I told you I found it in the third inning.”

Keller’s night was over, and the Pirates scored two more runs in the sixth to give their bullpen a four-run cushion.

It disappeared in a hurry.

Hartlieb, who worked his way into a high-leverage role for the injury-depleted Pirates bullpen, trotted in to begin the seventh inning. He walked the first two batters he faced, then hit Yadier Molina in the left wrist. The no-hitter was alive, but the game hung in the balance.

“He’s just scattered the ball. He has not been in the zone,” Shelton said. “It was something he did such a good job of previously. We have to get him back in the zone.”

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Out went Hartlieb, and in came lefty Sam Howard. Tyler O’Neill pulled Howard’s second pitch down the left-field line for a double, ending the combined no-hit bid and cutting the Pirates’ lead in half. Dylan Carlson then hit a sacrifice fly to right, Howard plunked Harrison Bader and Kolten Wong tied the game with a single to center field. Tommy Edman, the eighth batter of the inning, singled past a diving Erik González to put the Cardinals on top.

“When they got into their ‘pen and I came up with a couple of guys on there, I really felt like I had to do my job and get it in play and put it hard somewhere,” O’Neill said. “[Keller] had good stuff tonight. Luckily, we were able to get into the ‘pen.”

Right-hander Chris Stratton finished the inning by walking Paul Goldschmidt then striking out Miller and retiring Paul DeJong. In all, the three Pirates relievers needed 47 pitches to allow three hits, walk three batters, hit two more and, finally, record three outs.

“Sam came in, and they found some holes. One of those innings,” catcher Jacob Stallings said. “Strat did a great job of holding them there and keeping it a one-run game. Unfortunately, we couldn’t take advantage of that offensively. It’s just one of those things, I guess. One of those innings.”