Madison Keys felt like she was "in the zone" as she produced a brilliant display to crush Coco Vandeweghe in the US Open semi-finals.

The American reached her first grand slam final after a 6-1 6-2 thrashing of countrywoman Vandeweghe on Arthur Ashe Stadium on Thursday.

Keys, the 15th seed in New York, was in scintillating form from the outset, and finished with 25 winners and just nine unforced errors.

The 22-year-old, who required treatment on her right leg during the second set, said she quickly felt her game was near its best.

"I played really, really well. It was kind of one of those days where I came out and I was kind of in a zone, and I just kind of forced myself to stay there," Keys told a news conference.

"I knew I was going to have to play really well in order to beat her, and I feel like once things started going, it just kind of fell into place. Luckily I was able to close it out the way that I did."

Keys – who faces Sloane Stephens in the final – insisted the medical attention she received was called for purely as a precaution.

"I felt it at the end of my match the other day, and then I just didn't want it to become something that would be bad," she said.

"So as soon as I kind of felt it get the tiniest bit worse, I just had it wrapped to try and prevent anything from happening."

Vandeweghe, beaten for the third time in as many meetings with Keys, lauded her opponent and said she felt powerless.

"None of it had anything to do with the occasion. It was more Madison played an unbelievable match," the 20th seed said.

"I didn't really have much to do with anything out there."