SEATTLE — The Royals quickly squandered a chance at their first four-game win streak of the season, when a runaway first inning for the Mariners helped give Seattle the cushion it needed in an 8-2 win on Wednesday at T-Mobile Park.

Brad Keller was tagged for a season-high seven runs — four of which came from his first four baserunners — on nine hits and two walks over four innings, succumbing to a lack of command that he said eluded him while warming up in the bullpen.

“It just felt like I really didn’t have anything behind the fastball today,” Keller said. “It’s not a good feeling when you’re going into a start like that. So I was just trying to create stuff and threw myself out a little bit of a whack, and I left a few balls over the middle of the plate that they took advantage of. Just whenever I felt like I didn’t have my fastball, I was just trying to manipulate and it wasn’t working.”

Keller’s outing snapped a streak of five straight starts in which he’d gone at least six innings, over which he compiled a 2.83 ERA (though the Royals lost four of those starts). The pitching staff collectively had pitched solid over its 10 games prior to Wednesday, compiling an American League-best 2.97 ERA in that stretch.

When Keller is pitching well, Royals manager Ned Yost said, he is pumping his fastball low in the zone and complementing it with a plus slider. According to Statcast, Keller’s slider had a .160 batting average against entering Wednesday, well above the .216 MLB average, and a 30.1 percent whiff rate. The collective result typically yields soft contact and quick outs, not the product he was showing on Wednesday.

With location an issue, Keller left pitches over the plate out of the gate that the Mariners took advantage of. After giving up a double to Mallex Smith and walking J.P. Crawford, Keller served up back-to-back homers to Domingo Santana (on a first-pitch slider that caught too much plate inside) and Daniel Vogelbach (on a two-strike fastball that bled back onto the black).

“First inning, he really struggled to get the ball down, and [it was] just one of those days where he just battled,” Yost said. “He just couldn’t find his slot where he could consistently get down or down and away in the zone. His slider caught too much of the plate to Santana for a homer. It was total command. He couldn’t get the ball down.”

Keller recovered to strike out five straight, including the side in the second inning, but he also allowed his first four batters to reach in the fourth, which ballooned his pitch count to 95 and forced Yost to turn to his bullpen early, which the manager also had to do on Sunday and Monday.

Long reliever Brian Flynn pitched four innings of one-hit ball, though the one hit went for a solo homer to Santana, who put an impressive swing on a low-and-in two-seamer to inside-out the pitch 399 feet to right-center field. Flynn now has a 1.32 ERA over five outings since being activated off the injured list on May 25.

Yost said that with a bullpen he had to turn to over 9 1/3 innings on Sunday and Monday, only Flynn and right-hander Kevin McCarthy were available, at least in the context of how competitive the game was.

“He saved our staff,” Yost said. “It was a phenomenal job by Brian Flynn. We were really, really short, and I could tell early that we were probably going to be in trouble. I told [pitching coach] Cal [Eldred] that we were going to get through this game with Flynn and McCarthy, and after that, I’m pitching a position player.”

Yost anticipates that the Royals’ relief staff will be at full strength when they open a four-game series against the Twins on Thursday at Kauffman Stadium.

Keller’s first inning created too wide a deficit for an up-and-down Kansas City lineup to overcome, particularly with left-hander Marco Gonzales effectively changing speeds while limiting the Royals to just six hits and two runs over 6 2/3 innings.

One positive for the Royals was that those vying for playing time — third baseman Cheslor Cuthbert, shortstop Nicky Lopez and right fielder Jorge Bonifacio — each had two hits in the game.