Serena Williams’ 186-week reign as world No.1 is over after Karolina Pliskova beat her 6-2 7-6 (7-5) in the semi-finals of the US Open.

The 10th-seeded Czech moves into the final at Flushing Meadows to meet either Angelique Kerber – who will replace Williams as the world’s top-ranker player – or Caroline Wozniacki.

With the defeat, Williams’ bid to break the professional era record for career grand slam titles came to a halt.

For Williams, whose 22 grand slam titles has her tied with Steffi Graf, it marked the second consecutive year she lost in the semis while chasing history.

Williams came into last year’s semi-finals as the three-time defending champion and two wins shy of a rare calendar-year grand slam but was upset by Roberta Vinci.

She clutched at her left hamstring occasionally in the second set and double-faulted to end it.

The loss also prevents Williams from earning her US Open crown.

Pliskova advances to her first grand slam final.