American Serena Williams has blamed neither fatigue nor her injured left knee as the cause of her defeat to Czech Karolina Pliskova at the US Open tennis semi-finals.

"I’m not going to sit here and make an excuse", Williams told reporters after losing to Pliskova 2-6, 6-7 (5) here on Thursday, reports Efe.

As a result of the loss, Serena had to relinquish her number one ranking with German Angelique Kerber.

Serena said she had "serious left knee problems", which she thought occurred during a second or third round match.

"I wasn’t able to move the way I wanted to move, and my mind was just a little bit everywhere," said Williams, who has been competing in tennis for 20 years.

She also denied that the quarter-final match the day before, when she won against Simona Halep, left her tired for Thursday’s match.

"I wasn’t tired from yesterday’s match," she insisted and stressed that "If I can’t turn around after 24 hours and play again then I shouldn’t be on tour."

She simply attributed her defeat to the great play of her tenth-ranked rival, Karolina Pliskova, highlighting the good serves by the Czech player.