NEW YORK -- Serena Williams was upset in the U.S. Open semifinals for the second year in a row, beaten 6-2, 7-6 (5) by 10th-seeded Karolina Pliskova of the Czech Republic on Thursday night.
Williams, who clutched at her left leg between points in the second set, double-faulted to end it.
Afterward, her coach, Patrick Mouratoglou, said Williams injured her left knee in the quarterfinals.
The loss prevents Williams from earning her seventh championship at Flushing Meadows and 23rd major title overall, which would both have been Open-era records.
It also means Williams' 3½-year reign at No.