The American began his post-Wimbledon hardcourt run with a Washington semi-finals, which he lost to Fritz, a run to the final in the Montreal Masters, and now his best showing at Cincinnati - all with the US Open start barely more than a week away.
Nakashima withstood 22 aces from Fritz - who also bashed 31 unforced errors.
Nakashima trailed a break in the third set but broke back to knot it at 3-3 and broke again for a 5-3 lead.
He out-hit the powerful Fritz and converted on his first match point with a forehand winner.
"I've gotten a lot of confidence over the summer," Nakashima said. "In the final set, I trusted my game; I'm proud with how I ended the match.
"Today I was trying to hit bigger against him than in Washington; that's what got me through.
"I've had a pretty good last few weeks; I’ve spent hours and hours on the court - I'm proud of myself," he added. "I now have the opportunity to win more matches."
Nakashima became the first American men's player since 2011 to reach the semi-finals in both Canada and Cincinnati in the same year.
Check out the full match results here.
Nakashima will face either Lorenzo Musetti or Arthur Fils for a place in the final.