For the second straight year, Serena Williams saw her bid for a seventh US Open title halted in the semifinals with a 6-2 7-6(5) loss to Karolina Pliskova on Friday (NZ time) that also ended her long reign as world number one.

A year ago, Williams went into the semifinals on the cusp of a rare calendar-year grand slam but was the victim of one of the biggest upsets in tennis history when Roberta Vinci, a 300-to-1 longshot, prevailed.

This year it was Czech 10th seed Pliskova, who until this week had never played beyond the third round of a grand slam, delivering the upset as she swept Williams off an Arthur Ashe Stadium court she claims as her own in one hour, 26 minutes.

"I don't believe it. Actually, I do believe it," Pliskova said in a courtside interview. "I always knew I have a chance to beat anybody if I'm playing my game.

Karolina played great today. I think if she had played any less, then maybe I would have had a chance," Williams said. 

"So I think I wasn't at 100 per cent, but I also think she played well. She deserved to win today."

The 10th-seeded Pliskova began her on-court interview by blurting out that she couldn't believe she'd eliminated Williams to earn a spot in her first major final.

Then Pliskova changed course, saying:

"I mean, actually, I do believe it. I always knew I have a chance to beat anyone if I'm playing my game."

Her opponent in the final will be new world No 1 Angelique Kerber, who reached her third major final this year after dispatching Caroline Wozniacki in straight sets.

Kerber earned her first career No 1 ranking with Williams' loss.

Then German then went out and looked like a top-ranked player, winning the first four games en route to a 6-4, 6-3 victory.

Kerber had never made a Grand Slam final at the start of this year. Then she not only advanced that far but won the title at the Australian Open, beating Williams in the championship match. She followed that up with a run to the Wimbledon final, where she lost to Williams.

”‹At the 2015 US Open, Williams' bid for a calendar-year Grand Slam ended when she lost in the semifinals to unseeded Roberta Vinci of Italy in one of the biggest surprises in tennis history.

This one goes pretty high on the list, too.

This was the 33rd major semifinal of Williams' career, and the first for Pliskova, who beat the 34-year-old American's older sister Venus in the fourth round. Pliskova is only the fourth woman to beat both Williams siblings during the same Grand Slam tournament.