The No. 15 seed raced to a 5-0 lead and won 19 of the first 22 points, eventually winning the set in 23 minutes.
Keys reaches her first grand slam final appearance.
For the first time since 1981, the US Open women's semi-finals will be an all-American affair with seven-time Grand Slam champion Venus Williams and three first-Slam-title hopefuls in the mix.
A first-time career Slam champion is assured with Keys and Vandeweghe also having gone no deeper than a Slam semi-final. They roared for Williams during introductions after politely applauding Stephens.
Vandeweghe smacked 24 winners, a lot of them coming from her powerful forehand, while rocketing five aces and saving five of her seven break points.
Pliskova was beaten 7-6 (4) 6-3 by American CoCo Vandeweghe to end her hopes of clinching a maiden grand slam title.
Williams vs. Stephens was a back-and-forth affair, with a pair of lopsided sets leading up to a classic third. It is also worth noting that Keys did not face a single break point at all. Here's how long and successful her career has been: Williams' first title match in NY came in her U.S. Open debut in 1997.
Now she's playing in the US Open semi-finals.
The two sets lasted just 31 and 39 minutes, respectively. "We played junior tournaments together".
Sloane Stephens took the first set 6-1 against a mistake-prone Venus Williams in the U.S. Open semifinals. She finished with six double-faults and a total of 51 unforced errors, 24 more than Stephens.
The big-serving Czech had previously said that being number one came with additional pressure to perform at the tournament, where she lost in the final past year to Angelique Kerber.More news: Hamas Leadership Meets in Cairo
She was the junior champion in NY in 2008, but never had success in the main draw until now: Of her previous eight appearances, half ended in the first round, half in the second. Williams was the first, in 1997.
For the first time in 36 years, all four women to make it to the semifinals of the event are American. She is fed up of hearing that American tennis - which has not had a male grand-slam champion since 2003 - is over-reliant on the Williams sisters.
Madison Keys faced no such test, overwhelming CoCo Vandeweghe in the other semifinal.
U.S. Open semifinalist Juan Martin del Potro and two-time Wimbledon champion Petra Kvitova have won the U.S. Open sportsmanship awards.
Federer faces Del Potro for the first time in NY since the towering Argentine rallied from a set and a break down to shock him in the 2009 final.
"She's a great player, a great person".
The U.S. Open is the only Grand Slam tournament where Federer and Nadal have never played each other.
U.S. Open wheelchair tennis was played on Arthur Ashe Stadium on Thursday for the first time. Federer grabbed the second set 6-3. "We have so many Americans to talk about in the last days of the U.S. Open", Keys said. Keys next earned a tight service hold in which Vandeweghe managed to get to deuce twice, but was unable to earn a break point as the 15th seed held on to consolidate the break. Where Clijsters had already won the US Open in 2005, Stephens - who is 24 - had given the impression of an unfulfilled talent.
On the other half of the draw, the 12th-seeded team of Horia Tecau of Romania and Jean-Julien Rojer of the Netherlands eliminated No. 1-seeded Henri Kontinen of Finland and John Peers of Australia 1-6, 7-6 (5), 7-5.
In 1981, Americans Barbara Potter, Chris Evert, Martina Navratilova and eventual victor Tracy Austin reached the final four.