Third seed Nishikori failed to hit the heights that took him to the US Open semi-finals last month but still dominated his Japanese Davis Cup teammate Yuichi Sugita.
Local hero Kei Nishikori began his quest for a third Japan Open title by thrashing countryman Yuichi Sugita 6-4, 6-1 to reach the last 16 on Monday.
Third seed Nishikori failed to hit the heights that took him to the US Open semi-finals last month but still dominated his Japanese Davis Cup teammate, completing a one-sided victory with a fizzing backhand down the line after 81 minutes.
"I hoped I wouldn't have to play a Japanese player," said Nishikori, who last faced Sugita when they were juniors. "It's a relief to win. It took me a while to get into it," added the world number 12, who won the last of his 11 career titles in Memphis two years ago.
"I made a few too many mistakes at the start. But I tweaked what I needed to and enjoyed the second set a bit more. Hopefully I can go deep into the tournament. Of course, the aim is to try to win it."
Earlier, three-time Tokyo finalist Milos Raonic crushed last year's runner-up Adrian Mannarino, the sixth seed taking a little over an hour to beat the Frenchman 6-3, 6-4. The big-hitting Canadian, who has slipped from number three in the world to his current ranking of 20th after a poor run of form, fired 17 aces in a dominant display.
Canada's Denis Shapovalov, who last year made unwanted headlines after smashing a ball at an umpire and fracturing his eye socket in a Davis Cup tie, bundled out seventh seed Chung Hyeon of South Korea 6-3, 3-6, 6-2.
Elsewhere, France's Benoit Paire beat Chile's Nicolas Jarry 6-4, 7-6 (7/4) while Japanese qualifier Yosuke Watanuki upset Dutchman Robin Haase 6-7 (7/9), 6-4, 6-1. World number six Marin Cilic heads the field at the Tokyo tournament, whose list of previous winners include Roger Federer, Pete Sampras, John McEnroe and Ken Rosewall.
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