SEATTLE — June was the worst month in Kansas City’s 50-year history, as it dropped 21 of 26 games and fell to 20 games behind the American League Central-leading Indians.

Brad Keller did all he could to get the Royals off on the right foot to start July, and despite his best efforts, the rookie right-hander couldn’t outduel Seattle ace James Paxton.

Keller allowed just one run in a complete-game effort, but the Royals were blanked by Paxton in a 1-0 loss Sunday at Safeco Field.

Coming off his first career win as a starter, Keller (2-3, 2.09 ERA) gave up six hits and struck out three. But Paxton (8-2, 3.39) was at peak performance, giving up two hits and striking out 11 over eight innings as the Mariners earned a series sweep.

“Coming into the ballgame, I knew it was going to be a low-scoring ballgame,” Mariners manager Scott Servais said. “Their guy is really good. I think he’s first or second in the league in ground-ball rate. It was going to be a challenge for us, offensively, but luckily we had the Big Maple on the mound.”

Keller made 21 appearances out of the bullpen this season before making his first start May 30. The rookie right-hander threw seven scoreless frames in his last outing, a 2-0 win over the Angels on June 25.

“My sinker was working really well,” Keller said. “Anytime I fell behind in the count I just went to it. Really tried to jam those guys inside and it worked out pretty well. Got a lot of ground balls again today to the left side of the infield.”

Keller has made three straight quality starts, becoming a bright spot in a year full of woes for the Royals, who fell to 1-5 against the Mariners this season.

“I just see him stepping on the mound pretty much in control,” Royals manager Ned Yost said of Keller. “He’s out there attacking with his fastball and slider, throwing strikes, commanding his pitch count, doing everything you want a starting pitcher to do.”

Three straight singles by the Mariners with one out in the second provided the game’s only offense. Kyle Seager began the stretch with a line drive to right field. Ryon Healy followed with a single up the middle, before Ben Gamel put Seattle ahead, 1-0, with a broken-bat single to short center.

The Royals put only one runner in scoring position against Paxton. Salvador Perez led off the second inning with a double down the left-field line. Jorge Bonifacio followed with a walk, but both stayed put as Paxton struck out Hunter Dozier and Alex Gordon, and got Alcides Escobar to fly out to center.

A one-out single by Whit Merrifield in the third was the final hit of the day for the Royals. Kansas City has scored four or fewer runs in 22 of its last 23 games.

MLB saves leader Edwin Diaz struck out the side to close the door on the Royals in the ninth, notching his 32nd save of the season.