Frances Tiafoe believes the "sky's the limit" for him after reaching the French Open quarterfinals for the first time on Sunday.

The American downed German world No. 66 Daniel Altmaier 6-3, 6-4, 7-6 (7/4) in the Last 16 to set up a match with Lorenzo Musetti for a semifinal place.

It will be Tiafoe's first ATP quarterfinal since finishing runner-up at a low-key event in Houston in March.

"It's been horrible. I've been traveling around, and it hasn't been fun at all," he said.

"Kind of faced it. After that it gave me space to just play. You know, when I can play, I can play. This is not really something that's like a shock.”

"When I'm playing well, I'm there and present and having fun, I think the sky's the limit for me."

The 27-year-old, who has reached at least the quarters at the last three US Opens, is yet to drop a set in Paris and is confident he can go even further.

"I've been the super dark horse. I don't think anyone is really thinking that I was going to even be in this position," Tiafoe added.

"But at the same time, I don't really look at it like house money.”

"Now that I'm here, I'm like, 'Let's win'. I was having dinner yesterday. I'm like, 'Yeah, feeling great, but let's keep winning.' There's nothing more fun than winning.”

"Once I get my feet going and match under me, I'm dangerous. Then I'm feeling really good, and I'll play with anybody."

Tiafoe, seeded 15th at Roland Garros, has five wins over top-10 players at Grand Slam events.

"If you look at the guys in front of me, I'm much more comfortable playing the guys around me and ahead of me than the other guys," said Tiafoe, who is yet to drop a set in the tournament.