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 will have to relinquish her number one ranking with German Angelique Kerber set to replace her.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.