Serena Williams came into the first semifinal with a lot on the line. She hoped to remain the world no. 1 and pass Steffi Graf with her 23rd major title. But Karolina Pliskova had other ideas as she upset Williams in Arthur Ashe Stadium 6-2, 7-6.

It took very little time before the two big hitters put their powerful serves on display. With two players who consistently look for the big first strike, the points were quick and decisive. But it was Pliskova who found the first crack in Serena’s serve, gaining an early break. Playing with a quiet confidence, the Czech was all business and was able to protect her serve and added another break later, which sealed the first set. Her impressive serving exhibition led to an unlikely 26-minute set.

In the second set, Williams clearly attempted to elevate her intensity, pressuring the 10-seed. But despite a few nervous moments, Pliskova leaned on her serve to mute any threats. Meanwhile, Serena’s unforced errors piled up. Perhaps flat after her grueling quarterfinal match against Simona Halep, Williams lost her serve again as Pliskova moved to 3-2. But the advantage was short-lived as the 24-year old followed with a nervous serving game and the two players returned to equal ground.

With the New York crowd sensing that Williams was in a fight, they began to increase their volume in support.  However, Serena began to look noticeably gimpy and appeared as though she was favoring her leg.

Nonetheless, both players would protect their service games and the set advanced to a tiebreak. There, Pliskova raced out to a 3-0 lead as Williams sprayed a series of groundstrokes. But despite grabbing her leg during stoppages of play, Williams would win the next 4 points on the heels of solid defense and clutch winners. However, the Czech would work the tiebreak to a 6-5 advantage and as Williams served to stay in the match, she double-faulted to end the match and her tournament.

With the loss, Williams’ streak of 186 weeks at world no.1 came to an end and she will now have to wait until the Australian Open in January to pursue her 23rd major title. It is her second consecutive semifinal loss at the US Open. Angelique Kerber will become the world’s top player when the next rankings come out next week.

After the match, coach Patrick Mouratoglou confirmed that Williams was only able to practice at a minimal level due to a knee injury which limited her movement on court. “She was not moving at all today,” he said. “There was no match.”

But Pliskova deserves credit, she kept her poise despite the occasion and the pressure Williams attempted to apply. “I don't believe it,” Pliskova said afterward. “Actually, I do believe it. Serena is a champion. It's never easy to play her. I'm really happy that I stayed there to get the win.”

Pliskova will face the winner of the second semifinal between Angelique Kerber and Caroline Wozniacki.