ARLINGTON -- In just five weeks of work this season, Royals left-hander Danny Duffy has navigated through dozens of dominant innings, and he seemed well on his way on Friday night before running aground suddenly.

On his way to a 6-2 loss to the Rangers, Duffy faced only one batter more than the minimum in the first five innings, striking out five of the first 16 hitters. But he issued a leadoff walk in the sixth before giving up three consecutive singles, a run-scoring walk and a devastating grand slam to Joey Gallo.

“I feel like I let my team down tonight, but I’ve got a quick memory and I’ll keep doing what I’m doing. ... Hopefully there’ll be a lot less innings like that in the near future,” Duffy said.

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In the pivotal six-run sixth, Duffy walked Danny Santana, allowed a seeing-eye single up the middle to Shin-Soo Choo, gave up a line-drive single to Elvis Andrus and just missed throwing out Hunter Pence, who reached for a single on a grounder back to the mound. Nomar Mazara walked to score a run, and then Gallo launched a grand slam to deep center that traveled a projected 457 feet, according to Statcast.

“I left a ball out over to Gallo and he hit a big boy homer,” Duffy said. “There’s nothing I could’ve done any differently outside of not loading the bases. Maybe get the ball over to first base [on Pence’s grounder] a little bit quicker, but Pence was booking out of the box. It was just a disappointing sixth inning. I was cruising right along, and I’ve got to erase that and get onto the next one.”

The loss was Duffy’s worst of the season, as he allowed six earned runs in an outing for the first time since last August. He was unbeaten with a 2.73 ERA over his previous five starts before Friday’s setback.

“I’m not at all pleased with the result, [but] I’m proud of my body of work, and I’m going to continue down the path that I’m on,” Duffy said.

Gallo also acknowledged how effective Duffy has been this season -- the lefty beat the Rangers in Kansas City on May 14 -- and said Duffy was better on Friday than he’ll likely get credit for.

“His line’s probably not going to look great, but he pitched amazing,” Gallo said. “From the first at-bat, I said, ‘He’s going to be tough today.’ So we knew we were going to have to fight. You run into a guy who’s throwing that well, you’ve just got to hopefully get a big inning like we did.”

Before the sixth, Duffy was so efficient that manager Ned Yost was already thinking about how he was going to handle the late-inning bullpen decisions.

“He was rolling,” Yost said. “He was really doing a good job of handling his pitch count. And I’m thinking, OK, he’s going to waltz through the sixth, hopefully, and get him to the seventh, and my decision is going to be, is he going to be all right to go back out in the eighth?”

Prior to Friday’s loss, the Royals won four of the five games Duffy started in May while going 6-16 in the rest of the month’s contests. They scored only 121 runs in 28 May games (4.32 per game), almost a half-run below the American League average.

Kansas City’s offense didn’t generate much off Rangers starter Ariel Jurado. Third baseman Cheslor Cuthbert, called up earlier from Triple-A Omaha on Friday to replace struggling infielder Chris Owings, hit a solo homer in his first at-bat back with the big league club, but Alex Gordon’s RBI triple was the only other run scored off Jurado.