Kyrgios showed no signs of rust at the Citi Open after reaching his first major final at Wimbledon last month, breaking his Japanese opponent's serve in the opening games of the first and second sets.
He again broke Nishioka to win the match, after winning more than three-quarters of his own service points.
It was his first singles title since winning the Washington hard court tournament three years ago; he has risen to a new level of form in 2022 after years of battling mental demons.
Earlier in the tournament, he saved five match points and outlasted American Frances Tiafoe (24) to reach the semi-final round.
Kyrgios seemed overcome with emotion as he slumped to his back on the court after sealing the win for his seventh career singles title.
"It’s very emotional for me. To see where I was at last year to now it’s just an incredible transformation," he said. "I’ve shown some serious strength to just persevere and continue and get through all those times and to still be able to perform."
"There are so many people who have helped me get there, but myself, I've shown some serious strength to just continue and persevere and get through all those times and be able to still perform and win tournaments like this one."
Kyrgios, well known for his on-court antics and outbursts, opened up on his mental health struggles earlier this year, saying he had suicidal thoughts and the time around the 2019 Australian Open was one of his "darkest periods".
He said last year he was unsure how long he would continue playing professional tennis but he has shown a new level of consistency this season.
Kyrgios did not drop serve in Washington and has won 11 of his past 12 matches, losing only to Novak Djokovic in the Wimbledon final.
"Before Australian Open, I decided that I really wanted to put in a good training block, and then things just started falling into place," said Kyrgios, whose ranking rose to 37th.
"I had a great Australian Open. I felt like the tournaments after Australian Open I was kind of reinventing myself a little bit on the court.
"I feel like my motivation is a lot higher than it used to be ... I don't know. I just feel like there is a small window I should capitalize on it."
He also picked up a third doubles title of the year, returning to the court with Jack Sock (29) later on Sunday, August 7th, to defeat Ivan Dodig (37) and Austin Krajicek (32) 7-5 6-4. Kyrgios won the Australian Open and Atlanta doubles titles with countryman Thanasi Kokkinakis (26).
Kyrgios will be hoping to extend his current run of good form into this year's US Open, which starts at the end of August.
