"Basically, it was along the lines of 'Swing away. Knock it out if you can,'" Karns said.
And that's what Karns did, re-routing a first-pitch fastball from Phillies starter Aaron Nola into the left-field stands for a home run that turned out to be the only run in the Rays' 1-0 win over the Phillies on Tuesday night.
"I saw a great pitch, swung, barreled it and it went out," Karns said. "So I mean, I wasn't really trying to do that. But I'll take it.
" ... I really wasn't sure if it was going out. I was completely just kind of caught off that I hit it. And then I took off, and it kept going. It was a great feeling rounding second. I looked into the dugout, they're all standing up doing crazy stuff. It was definitely a memory I'll never forget."
Karns' blast led to the first Major League game in which a team won, 1-0, on a solo home run by a pitcher since April 29, 2009, when Yovani Gallardo homered in the Brewers' 1-0 win over the Pirates.
Karns became the first AL pitcher to hit a home run to account for a game's only run since 1962, when Milt Pappas did so for the Orioles against the Yankees. And there has never been a game in the history of Interleague Play where only one run scored on a home run by an AL pitcher.
Karns joined Esteban Yan as the only pitchers in Rays history to hit a home run.
"It's just one of those funny things you find yourself a part of sometimes," Karns said. "I'm going to take pride in that and continue on with my career and hope that it's a long one."
Karns' first home run since high school was also the first hit of his Major League career. He was 0-for-2 this season and 0-for-3 in 2013 for the Nationals.
Yan homered off Mets right-hander Bobby Jones in the second inning of a 15-5 Tampa Bay win on June 4, 2000.
In a precursor for Tuesday night's accomplishment, Karns put on a power display in Toronto while taking batting practice Sunday afternoon. Kevin Kiermaier happened to be hitting in the group with Karns, much to the outfielder's chagrin.
"I said, 'I'm never taking BP with you again, man,'" Kiermaier said. "Because I was sitting here trying to hit a couple out. And he's going second deck in Toronto. He did it three times in a row.
"I'm like, 'Cool, Karns hasn't picked up a bat in weeks, and he's sitting here launching balls in BP. I'm sitting here trying to feel good about myself.' ... But it's awesome."
Not to be forgotten, Karns picked up his fifth win of the season, but he cited the work of the training staff for helping him fight a battle against dehydration for all of his five innings. During the course of that time, he went through two jerseys, four undershirts and two hats, which he soaked with sweat.
Karns' home run brought the Rays' staff its first hit since Alex Cobb doubled at St. Louis on July 23, a period covering 23 at-bats.
Cash was asked where Karns' feat ranked on the list of unexpected things this season.
"That might have been the top," Cash said. "If you would have told me we would have won a 1-0 ballgame with Nathan Karns hitting a home run, that's probably at the top."
Added Phillies catcher Cameron Rupp: "You've got an American League pitcher up there, it's the first pitch of the inning. You're not going to try to trick anybody. I've known Nathan for a long time, and I can sit here and say a blind squirrel finds a nut every now and then."
Bill Chastain is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.