KANSAS CITY — A six-run lead and an apparent cruise-control victory for the Royals on Sunday suddenly went awry when a rare error by shortstop Adalberto Mondesi led to three unearned runs in the sixth inning and when closer Ian Kennedy gave up four hits and three runs in the ninth trying to protect a three-run lead.

The Royals, though, came away with a dramatic, 8-7, walk-off victory in the 10th inning when Billy Hamilton walked, stole second base, and scored on Whit Merrifield’s bad-hop single off third baseman Gio Urshela with two outs.

The Royals have been looking for a good break for quite some time.

“For sure,” Merrifield said. “I always say, ‘Better lucky than good.’ The hop went our way. Nice to have one bounce our way.”

Merrifield’s grounder did have some serious topspin on it before hitting a divot.

“Especially later during a day game, the sun is beating down all day, the ground gets pretty hard,” Merrifield said. “Shout out to [head groundskeeper] Trevor [Vance] for putting a clog of dirt there.”

It was Merrifield’s fourth career walk-off hit.

Royals manager Ned Yost noted the key plate appearance came from Hamilton, who drew a walk off Jonathan Holder.

“A key walk there and he didn’t get too anxious,” Yost said. “Took a 3-1 strike as we had the take sign. Didn’t get overanxious on the 3-2. And then he was in go mode after that.”

The Royals had a chance to win it in the bottom of the ninth, when they loaded the bases with two outs. But Ryan O’Hearn popped up to second.

But the Royals’ 10th-inning rally saved what would have been an unsightly defeat.

“Ian has been solid for us all year,” Merrifield said. “For that to happen … earlier in the year, we gave stuff up and found a way to lose. So it was nice to win one.”

Meanwhile, there apparently is something about Yankees rising star right-hander Domingo Germán that brings out the best in the Royals.

German, who leads the Majors with nine wins, has only one loss this season — on April 18 against the Royals at Yankee Stadium when Kansas City hit two home runs.

And prior to Sunday’s game, the Royals had hit only three homers in their previous 12 games. But they tagged German with four home runs in the first five innings.

“There’s a couple of guys that the league has pounded on and we can’t do anything off them,” Yost said. “Then we get to German, who is one of the best pitchers in the league.”

Left-hander Danny Duffy was the beneficiary of the Royals’ long-ball assault and seemingly was headed toward his fourth straight win. He went six innings and gave up five hits and four runs, only one earned. He walked one and struck out four.

Martín Maldonado got it going against German, ripping a curveball into the left-field seats in the second inning for a three-run home run. Maldonado only had one home run and three RBIs coming into the game.

O’Hearn, hitting .174 coming into the game, snapped a 0-for-15 spell with an opposite-field shot off German in the fourth.

The Royals really flexed their muscles in the fifth. After a checked-swing double by Mondesi, Hunter Dozier powered a four-seam fastball into the left-field seats and the Royals led 6-1.

A minute later, Jorge Soler jumped on a first-pitch four-seamer and sent it over the left-field fountains into the upper deck and the Royals were up 7-1.