The world number 11 has picked up two titles on tour already this season and arrives at the Citi Open in Washington having won 32 of his 40 matches this year.
He has beaten one of the top five, Roger Federer, this year at the Miami Masters, and also took Rafael Nadal to three sets in the Madrid Masters final before retiring with a back injury.
Titles in Barcelona and Memphis have seen him climb as high as number nine in the world rankings and, under the supervision of coach Michael Chang, he believes he can close the gap on the game's best players.
"There's still a big difference in the top five or so," said the 24-year-old. "They don't miss easy shots and don't give you easy points to win a game.
"For me, I don't feel any fear to play them any more after beating them a couple of times.
"I have more confidence now to play the top guys."
Nishikori returns for the hard-court event this week after resting following his fourth-round exit to Milos Raonic at Wimbledon last month and is keen to finish the year strongly.
"I feel very strong," he added. "Physically I'm ok.
"I had a great two weeks of training. Everything is ready for the US Open."