In a match that lasted more than 3 hours and 30 minutes, the Maryland native defeated Nadal 6-4, 4-6, 6-4, 6-3.
Tiafoe, 24, is the youngest American man to get this far in the US Open since a then-24-year-old Andy Roddick was a finalist in 2006.
“It was definitely one hell of a performance,” he said following the match. “I played really well today. Yeah, I guess, I mean, I just came out there and I just believed I could do it. It helps I played him a couple times.”
Earlier in the tournament, Tiafoe said he was happy to be going mostly unnoticed.
After his win Monday, he expressed joy for what his victory against Nadal meant to his parents, who immigrated to the United States from Sierra Leone.
“I just had a big passion for the game. Not even mainly for me, but to do it for them. To see them experience me beat Rafa Nadal, they’ve seen me have big wins, but to beat those Mount Rushmore guys, for them, I can’t imagine what was going through their heads.
“Yeah, I mean, they’re going to remember today for the rest of their lives.”
Nadal, 36, recorded his first loss in a grand slam event this year and is now 22-1 this season. The Spaniard won the Australian Open and French Open before withdrawing from Wimbledon in July before his scheduled semifinal showdown with Australian Nick Kyrgios.
“The difference is easy,” Nadal said afterward. “I played a bad match and he played a good match. At the end that’s it, no?”
Nadal said Tiafoe played more solidly than in previous matches between the two, served well, had good backhands and was quick.
“But I don’t think I pushed him enough to create him the doubts that I need to create,” he said. “Of course, he did the things well. If not, he will not win, without a doubt. Congrats to him.”
Nadal said he needs to “fix things” and when he feels mentally ready to compete again, he will, but he is unsure about the timetable.
Tiafoe will face Andrey Rublev of Russia in a quarterfinal on Wednesday.
The US Open men’s singles final is scheduled for Sunday.
Tiafoe’s parents met in the United States, where they would have twins, Franklin and Frances.
Their father signed on as a day laborer at the Junior Tennis Champions Center in 1999 in College Park.
While working around the clock, his father was forced to move into a vacant storage room at the tennis center, where his two boys would stay with him, sleeping there, while their mother worked night shifts as a nurse. Tiafoe picked up the sport, getting to play at the center for free, and fell in love with the sport.
Tiafoe previously said that while his family didn’t grow up wealthy, he wouldn’t change anything.
On Monday he told reporters: “Us being around tennis was kind of gets us getting out of our neighborhood. My dad kind of being able to watch us. It wasn’t anything supposed to be like this. Once we got in the game of tennis, it was like my dad was like, It would be awesome if you guys can use this as a full scholarship to school. I mean, we couldn’t afford a university.”