With his first major league season barely in the books, lefty Yusei Kikuchi is already mapping out plans for his 2020 season for the Seattle Mariners.

"I'm going back to the fastball," Kikuchi said recently.

Kikuchi went 6-11 this year with a 5.46 ERA after signing with the Mariners over the winter. The 28-year-old believes his best results came when he was able to establish the fastball.

There is an irony in this, since the lefty's career year came in 2017, when he had the most effective fastball of any pitcher who qualified for an ERA title in Japan -- according to data analysis website Delta Graphs.

In the belief that his slider would become his big pitch in the major leagues, Kikuchi focused more on that pitch starting in 2018, his last with Japan's Seibu Lions.

"If I'm able to pitch up to my abilities, I am absolutely going to get results," said Kikuchi, who is now doing some recovery work at the Mariners' spring training facility in Arizona.

"I feel that the biggest thing for me was being allowed to stay in the starting rotation all season, and now it is completely on me to make that investment in me pay off."

After a brief return to Japan, Kikuchi expects to get back into rigorous offseason training starting in November. Because, the lefty believes his real strength is his ability to train hard.

"I'm not a hamster, but if you put me on a treadmill and told me to go, I'd run for all I was worth," he said. "Doing the same thing over and over and keeping it up is what I'm good at. It's fun."