Tiafoe, 24, is the youngest American to be this far at the US Open since Andy Roddick, then 24, was a runner-up in 2006.
“It was definitely a hell of a performance,” he said after the game. “I played really well today. Yeah, I guess, I mean, I just got out and I just thought I could do it. It helps to have played it a few times.”
Earlier in the tournament, Tiafoe said he was happy to go mostly unnoticed.
After his victory on Monday, he expressed his joy for what his victory over Nadal meant for his parents, who immigrated to the United States from Sierra Leone.
“I just had a big passion for the game. Not even primarily for me, but to do it for them. To see them see me beat Rafa Nadal, they saw me get big wins, but to beat those guys from Mount Rushmore, for them, I can’t imagine what was going through their minds.
“Yeah, I mean, they’ll 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 the French Open before pulling out of Wimbledon in July ahead of his scheduled semi-final showdown with Australian Nick Kyrgios.
“The difference is easy,” Nadal said afterwards. “I had a bad game and he had a good game. In the end, that’s it, isn’t it?”
Nadal said Tiafoe played more solidly than in previous games between the two, served well, had good backhands and was fast.
“But I don’t think I pushed him enough to create the doubts in him that I need to create,” he said. “Of course he did it right. Otherwise he won’t win, no doubt. Congratulations to him.”
Nadal said he needs to “fix things” and will do so when he feels mentally ready to compete again, but is unsure of the timing.
Tiafoe will face Andrey Rublev of Russia in the quarterfinals 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 at 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 nurse. Tiafoe took up the sport, was able to play at the center for free, and fell in love with the sport.
Tiafoe previously said that even if his family didn’t become rich, he wouldn’t change anything.
On Monday, he told reporters: “Being around tennis kind of got us out of our neighborhood. My dad could kind of watch us. It wasn’t supposed to be like that. Once that we got into the game of tennis, it was like my dad was like, it would be awesome if you could use that as a full scholarship for school I mean, we couldn’t afford college.
CNN’s Jill Martin contributed to this report.
