Last year's runner-up Fritz is the sole American man left from the 23 who began the main draw and also the nation's last hope of ending a 22-year major drought since Andy Roddick held aloft the sterling silver trophy at Flushing Meadows in 2003.
"It's been a tough week for the guys," said Fritz. "I wasn't expecting that. I felt really good about our chances this week. I'm happy to be here and happy to be the last one standing. Hopefully, the crowd will get behind me and will get me through it."
The 27-year-old went toe-to-toe with Machac in the opening set, which he clinched with a break in the 10th game after the Czech hit a backhand wide, and dialled up the intensity in the eighth game of the next set at Louis Armstrong Stadium.
A second break handed Fritz the advantage and he quickly doubled his lead in the encounter with a near-flawless service game, leaving his jolted opponent facing a daunting task to reach his maiden major quarter-final.
"I had some battles in the last couple of matches," Fritz added. "I felt very dialed in tonight ... solid, aggressive at the right times."
The writing was on the wall when Fritz fired a swerving ace down the 'T' to go 3-1 up in the third set, and the fourth seed held firm to book a last-eight clash with either 24-times Grand Slam champion Novak Djokovic or German Jan-Lennard Struff.
"I don't care (who I play) ... I just want to go out and play my tennis," Fritz added.