Serena Williams revealed she battled a serious knee injury at the US Open as she was stunned by Karolina Pliskova in the semi-finals.

The American's 186-week reign as world number one is over after the 6-2 7-6 (7-5) loss to Czech Pliskova at Arthur Ashe Stadium on Thursday.

Williams, whose bid for a 23rd grand slam also came to an end, said a knee injury had been impacting her from the opening rounds of the tournament.

"I have been having some serious left knee problems," she said.

"I wasn't tired. Fatigue had absolutely nothing to do with it. If I was tired I should definitely get into a new career."

Williams added: "I wasn't able to move the way I wanted to move.

"When you're injured you're thinking of other things when you should be just playing and thinking of your shots.

"My mind was just a little bit everywhere. But it was what it was."

Williams, 34, refused to discuss the number one ranking, which she gives up to German Angelique Kerber.

She was also unwilling to blame the scheduling, having had to back up after her quarter-final win over Simona Halep the night prior.

"I wasn't tired from yesterday's match. I'm a professional player; been playing for over 20 years," Williams said.

"If I can't turn around after 24 hours and play again then I shouldn't be on tour. So I definitely wasn't tired from yesterday's match at all.

"It wasn't a five-hour match. I have practiced three hours, so it wasn't that huge of a deal."