"Kudos to [Aledmys] Diaz," Karns said. "He battled and got a pitch that he could do something with."
With two runners on and one out, Karns threw seven pitches to work to a 3-2 count on Diaz. Then he located a 93-mph fastball in what he said he thought was high enough to be a "safe zone."
It was not.
"He got on top of it and drove it out," Karns said.
That home run shrunk the Mariners' lead to 5-4. Karns got out of the inning without another run scoring. The bullpen covered the final four innings, ensuring Karns' first victory since another Interleague series last month, against the Padres.
Up until that inning, Karns had held St. Louis to a single run on two hits while his offense put five runs up on the board.
It was shaping up to be a solid start for Karns, who compiled a 7.36 ERA in his other four starts this month.
"I thought he was really sharp the first couple innings," manager Scott Servais said. "He's struggled a little the last couple times out. He'll have a couple good innings and then he kind of runs into trouble and struggles to get through it and that's where the pitch count goes crazy on him."
He totaled 97 pitches in five innings Saturday.
"I'm trying to figure out," Karns said. "I'm not really quite sure what's going on."
Maddie Lee is a reporter for MLB.com based in Seattle. This story was not subject to the approval of Major League Baseball or its clubs.