American 20th seed CoCo Vandeweghe reached the US Open semi-finals Wednesday with a 7-6(4), 6-3 win over Karolina Pliskova as the Czech player lost her world No 1 ranking to Garbine Muguruza.

Pliskova had been the runner-up in 2016 and her failure to defend those points means that Wimbledon champion Muguruza will take the top ranking for the first time despite the Spaniard losing in the last-16 in New York.

Vandeweghe’s victory means that the US Open will have an all-American semi-final line-up in New York for the first time since 1981 after 15th seed, Madison Keys defeated Estonian qualifier Kaia Kanepi 6-3, 6-3 on Wednesday. Earlier, on Tuesday, Venus Williams and Sloane Stephens had reached the semi-finals after winning their respective quarter-finals.

“I won the juniors here when I was 16 and dreamed of playing on the real stage,” said 25-year-old Vandeweghe, who also made the semi-finals at the Australian Open, beating then-world No 1 Angelique Kerber in the process.

The last time four US women were in the semis was 36 years ago when eventual champion Tracy Austin, Chris Evert, Martina Navratilova and Barbara Potter all made it.

Pliskova, who had to save a match point to beat China’s Zhang Shuai in the second round, shrugged off losing her top spot.

“I just don’t care,” she said. “I didn’t choose to be at this position where I am now. There are many more Grand Slams to come. I believe I can win one.”

Keys will be playing in her second Slam semi-final after the 2015 Australian Open.

“It means the world to me,” said Keys who underwent wrist surgery earlier this season.

“If someone had said after Wimbledon I would be here now I wouldn’t have believed them.”

Kanepi, ranked 418 but a six-time quarter-finalist at the Slams, had battled back from crippling foot injuries and a debilitating virus that has limited her to just five events in 2017.

She was unable to convert three break points in the third game of the opening set and there was no way back.

In Thursday’s semi-final matches, Keys will face 20th seed CoCo Vandeweghe, whom she is 2-0 against in WTA matches, while Stephens faces seven-time Grand Slam champion and two-time US Open winner Williams.