Madison Keys ensured an all-American semi-final draw at the US Open after outclassing Kaia Kanepi in straight sets.

Coco Vandeweghe stunned world number one Karolina Pliskova earlier on Wednesday to join countrywomen Venus Williams and Sloane Stephens in the final four at Flushing Meadows.

And 15th seed Keys rounded out a perfect day for the United States with a dominant 6-3 6-3 victory in New York.

It is the first time since 1981 that all four semi-finalists at the US Open are American as Keys prepares to face Vandeweghe.

"It's pretty cool," Keys said of having all-American semi-finals. 

It was a smooth start from Kanepi, who earned two early break point opportunities. But Keys dug herself out of a 15-40 hole and fended off another with an ace down the tee as she held in the second game.

With the support of a partisan crowd, Keys then claimed the one and only break for a 3-1 lead.

And Keys never looked back as she raced through the opening set in less than half-an-hour, highlighted by her five aces and 12 winners.

Kanepi huffed and puffed, saving two break points in the fifth game to stay alive in the second set.

However, she could not find a way past Keys' impenetrable serve, despite saving a match point and then earning a break point chance, as the latter reached the semi-final of a grand slam for the second time, having featured in the final four at the 2015 Australian Open.

STATISTICAL BREAKDOWN
Keys [15] bt Kanepi 6-3 6-3

WINNERS/UNFORCED ERRORS
Keys - 23/21
Kanepi - 8/16

ACES/DOUBLE FAULTS
Keys - 8/0
Kanepi - 0/2

BREAK POINTS WON
Keys - 3/8
Kanepi - 0/3

FIRST SERVE PERCENTAGE
Keys - 72
Kanepi - 67

PERCENTAGE OF POINTS WON ON FIRST/SECOND SERVE
Keys - 79/60
Kanepi - 65/38

TOTAL POINTS
Keys - 67
Kanepi - 50