"It's absolutely unbelievable to win in Japan," said Nishikori. "To win an ATP 500 event will give me more confidence in the future.
"I'd never played well in Japan but since beating (second seed) Tomas Berdych (in the quarter-finals), I felt I'd broken that wall down. To reach the top 10 is my target now."
Roanic, who hit 14 aces and will move up to 14th in the world rankings, said: "Kei is a superstar in Japan.
"I don't think I had a letdown after beating Murray. Kei is so quick and he was reading my serves well, which doesn't usually happen. I love coming to Japan and thanks Japan for inventing sushi. I love it."
