Kansas City, Mo. ? Danny Valencia’s own grandchildren may have trouble believing this story one day.

Not only was his first major-league home run a grand slam, it came in a 19-1 victory. And on a night he went 4-for-4. And, topper of toppers, it was off the reigning American League Cy Young winner in a six-run first inning.

The rookie third baseman, who didn’t even homer in the minors this year before being called up in early June, cleared the bases against Zack Greinke in the first inning of Minnesota’s rout of Kansas City on Monday night.

“What better way to start it off?” Valencia said with a big grin. “Not just hit a home run, but a grand slam, and off a guy who’s an accomplished big leaguer and won the Cy Young. It was great.”

Valencia didn’t know he’d gone into the record book as the first Minnesota Twin to hit a grand slam for his first major-league homer.

“Really? That’s awesome,” he said. “Pretty cool.”

Joe Mauer did even better on a night the Twins set a season high for runs and tied their season best with 20 hits. The AL’s 2009 MVP was 5 for 5 with a home run and a career-high seven RBIs.

“It was just one of those nights where everything we swung at started finding holes and going all over the place,” said Twins manager Ron Gardenhire. “A great night offensively for us. A lot of people involved all the way up and down. You don’t see that too many times.”

Mauer had three singles, a double and a three-run home run that made it 17-0 in the seventh. The five hits tied his career best.

The crowd booed in the eighth when Drew Butera pinch hit for him.

“I asked him if he wanted another bat,” Gardenhire said. “Do you want to hit? He said, ‘No, I’m good.’ That’s good enough for me.”

The Twins skipper is planning to give his All-Star catcher as much rest as possible the second half of the season.

Five Twins had multihit games. Their 11 extra-base hits included two home runs, two triples and seven doubles.