Danny Duffy has quickly become one of the best starters in the American League so far this season.

The Royals lefty kept the Tigers’ offense quiet in five scoreless innings Sunday afternoon at Comerica Park, guiding Kansas City to a 4-0 win and series win with its third consecutive victory over Detroit this weekend. The Royals have won nine of their last 12 games, and this weekend's success came on the strength of their pitching.

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Duffy’s season ERA is now 0.39 -- that’s one earned run in 23 innings -- in four starts, and the veteran has been the Royals’ most reliable starter this month. As Sunday’s games around the league wrapped up, Duffy led American League starters in ERA and was third in baseball behind the Mets’ Jacob deGrom (0.31) and the Brewers’ Corbin Burnes (0.37).

“It has been fun to be effective,” Duffy said. “I’ve been really inconsistent the last few years. I’ve had good stretches and really bad stretches. I think what I’m doing right now is just throwing everything with maximum intent. Believing in everything. I’ve got a manager that believes in me, two pitching coaches that are amazing. Our coaches top to bottom are great. Building you up and really making you believe that what you’ve got in the tank is enough.”

On Sunday, Duffy allowed just four hits, struck out eight and didn’t walk a batter for the first time this season. He only faced trouble once. After a leadoff single in the fourth, Niko Goodrum was caught stealing on an excellent throw from Salvador Perez. Then with runners on the corners and two outs, Duffy struck out Willi Castro swinging on a 95-mph fastball to end the inning.

In the past, it was common for Duffy to have those types of innings spiral on him. But he’s been able to work his way out of traffic with a calmer presence and belief in his stuff. Last week against the Rays, he got out of a bases-loaded jam in the sixth inning with a weak double play ball.

“He does a nice job of controlling what he can control,” Royals manager Mike Matheny said. “Doing a nice job of not getting distracted when he does get some pressure on, he just continues to make pitches. I think it comes back to what we talked about already, the fact that his mind is in a good place.”

Duffy relied on his curveball more than he typically does, using it for 18 percent of his pitches Sunday. None were put in play, and he got three swings and misses as well as a called strike on the pitch. He said he tweaked his mechanics with his curveball when playing catch with Brad Keller in between starts, and now he’s able to get his foot down earlier and change the plane as it comes in to a hitter. He threw it harder on Sunday than he has before -- one registered at 80.1 mph according to Statcast, while the pitch has averaged 77 mph this season.

“I felt better with my curveball than I think I’ve honestly ever felt,” Duffy said. “Maybe dating back to my rookie year when that was kind of my go-to pitch. … That’s why catch is so important. None of us are out there messing around with the way we approach our throwing program. It pays off.”

As the offense has manufactured runs on the basepaths the past two games -- their four stolen bases Sunday brought their total to an AL-best 22 -- the Royals’ pitching staff has shut down the Tigers. It allowed just three runs over the weekend, and Keller will get a chance to pitch the Royals to a sweep on Monday. After watching Mike Minor allow two runs in 5 2/3 innings with nine strikeouts on Friday, and Brady Singer dazzle with a seven-inning outing Saturday, Duffy was intent on keeping the line moving.

“It’s definitely a family,” Duffy said. “You see a dude just go right through a lineup like that, it’s something you want to keep going.”

“I love when a rotation starts to see it as healthy competition,” Matheny added. “They spend a lot of time together, and they’ve been doing a really nice job of watching each other’s ‘pens, talking shop even during the middle of the game when they’re not starting. You can tell that they’re paying close attention, trying to not just feed off what the other guy did, but learn from what the previous guy did.”

And the bullpen has built off the rotation’s strength, too. Kyle Zimmer, Jake Brentz and Wade Davis shut down the Tigers after Duffy’s five innings, and before Davis’ walk to Jeimer Candelario in the ninth inning, Royals pitchers hadn’t walked a batter since Friday. There were just three walks in that series-opening game.

“It’s been a lot of fun, and there’s no better team to be on right now,” said Brentz, who logged two innings for the first time in the Majors. “We just feed off each other. All our mindsets are just go out there and throw up a zero and get the team back in the dugout.”