Karolína Plíšková beat Serena Williams to end the American's reign as world number one ©Getty Images

Serena Williams has been dethroned as world number one by Germany’s Angelique Kerber after the American was stunned by Czech 10th seed Karolína Plíšková in the semi-finals of the US Open at Flushing Meadows in New York City.

Williams, chasing a seventh US Open crown, was outplayed during her last four encounter with the tricky Czech as she suffered a surprise 6-2, 7-6 defeat.

The result ensured Kerber, this year’s Australian Open champion, would overtake the 34-year-old at the top of the women’s rankings.

She becomes the first German woman to achieve the feat since Steffi Graf in 1996 when the standings are updated following the conclusion of this tournament.

The German set up a meeting with Plíšková in the final as she overcame unseeded Danish player Caroline Wozniacki 6-4, 6-3.

"To be number one in the world sounds amazing," Kerber, whose victory against Wozniacki saw her book her place in the finale of the US Open for the first time, said.

"For me it's amazing to be, after Steffi, the number one player from Germany.

"Steffi is a great champion, a great person and I think she's proud of me to be the next number one after her."

Germany's Angelique Kerber is the new women's world number one ©Getty Images
Germany's Angelique Kerber is the new women's world number one ©Getty Images

Williams, who had looked in superb form throughout the event, struggled against the big-serving tactics of Plíšková as the Czech began to take control early on.

Plíšková, who had never made it beyond the third round of a Grand Slam until the US Open, breezed to the opening set, with the American taking 50 minutes to earn a break point.

The second set at the Arthur Ashe Stadium proved to be a closer affair as Williams threatened a comeback, breaking the 24-year-old’s serve to give herself a chance of turning the match around.

But it turned out to be short lived as Plíšková booked her place in a first-ever Grand Slam final by taking the second on a tie break.

"I don't believe it," said Plíšková, who beat Serena’s sister Venus in the fourth round.

"I knew I had the chance to beat anyone if I played my game.

"I am excited to be in the final and to beat Serena as she is such a great champion.

"Even when she was down a set and break she was still fighting. 

"I had to fight hard to win."

The men’s semi-finals come to the fore tomorrow, with Kei Nishikori of Japan taking on two-time Grand Slam winner Stanislas Wawrinka of Switzerland.

World number one Novak Djokovic is due to face Frenchman Gaël Monfils.