ST. LOUIS -- Mitch Keller didn’t have his best stuff on a brisk, chilly Sunday afternoon at Busch Stadium. He assessed that his stuff wasn’t crisp. He didn’t have his best curveball, forcing him to rely on his cutter and sinker along with his fastball. The result: a quality start.

Keller threw six innings of three-run ball in the Pirates’ 5-4 10-inning loss to the Cardinals, the sixth consecutive quality start recorded by Pittsburgh’s rotation. Last year, these types of outings were difficult to come by. This year, they’re becoming the norm.

“That’s our job, to go as deep as we can,” Keller said. “It’s really cool to see that we’re finally putting it together here, all of us. I think it just comes back to really good game plans. Everyone is on a really good routine. We’re sticking with our plan and really getting good work in. The test will be to keep it going for the rest of the year and be consistent.”

This current stretch of pitching is, by far, the best of Derek Shelton’s managerial tenure. Over their past six games, Pittsburgh’s rotation has allowed 10 runs across 37 innings with 32 strikeouts to 12 walks. The last time the Pirates recorded at least six consecutive quality starts was from April 10 to April 19, 2019, when Pittsburgh’s rotation -- Joe Musgrove, Jameson Taillon, Jordan Lyles, Chris Archer and Trevor Williams -- combined for eight quality starts.

“Our starters have done a really good job,” Shelton said. “Very similar to [Roansy Contreras], Mitch battled through some stuff today. He wasn’t as sharp as we had seen him. It was cold. But really did a nice job of keeping us in the game.”

In ‘22, Pittsburgh’s starters combined to record 38 quality starts, the third-fewest in all of baseball. Keller, himself, recorded 13 of those quality starts. Only the Twins (35) and Nationals (30) had fewer. In ‘23, by contrast, the Pirates’ rotation is quietly among baseball’s best.

Pittsburgh’s starters have combined to throw eight quality starts in 16 attempts, tied with the Guardians and Rays for the second most in baseball and one behind the Dodgers for the most in baseball. Keller and Johan Oviedo are leading the way so far with three and two, respectively, while Contreras, Vince Velasquez and Rich Hill have one apiece. For Keller, in particular, quality starts are becoming the expectation; since June 2022, Keller has recorded 15 quality starts on 25 opportunities.

This early stretch is all the more impressive considering the Pirates are without right-hander JT Brubaker, who underwent season-ending elbow surgery to repair the ulnar collateral ligament (UCL) on Wednesday. Brubaker not only threw the second most innings for the Pirates last season (144), but recorded the second most quality starts (eight).

The rotation’s ability to pitch deep into games, in turn, allows the bullpen to remain fresh. Entering play, Pittsburgh’s relievers combined to throw 53 2/3 innings, which ranked 18th among all teams in baseball. During this series, the Pirates’ relievers threw a combined 11 2/3 innings, while the Cardinals' relievers threw a combined 15 innings.

“They're freaking pitching their butts off,” said Wil Crowe, who took the loss on Sunday. “It definitely helps us in the bullpen, because the less innings we pitch early, the more fresh we'll be late. I think that's what the good teams do. Their starters cover, and then when you get to August and September, your bullpen arms are still fresher and still electric and still doing what they've got to do. I don't think you see as much snowball effect early, but you see it later on when bullpen arms aren't as taxed as they could be.”

This rotation will be tested in the coming days as the team travels to Denver for a three-game set against the Rockies at Coors Field, a hitter’s paradise where the progress of Pittsburgh’s pitchers can perish. No starter wants to be the one who ends this current run. The streak will eventually end -- all streaks do. The question, then, is how long can it last?

“Day in and day out, as starting pitchers, we notice when other starters are going deep,” Keller said. “You gotta follow it. You gotta keep going. Everyone wants to keep following what the last one did and wants to out-do the other one. I think having that mentality is really good for our staff and anyone really.”