Defeated star also relinquishes world No 1 ranking to Kerber

Serena Williams couldn't get her mind off her sore left knee as she was sent reeling from the US Open semifinals on Thursday by 11th-ranked Czech Karolina Pliskova.

The 34-year-old Williams, who was gunning for a 23rd Grand Slam title that would take her past the record she shares with Steffi Graf for most in the Open Era, said "some serious left knee problems" had affected her game - as much mentally as physically.